diff --git a/pox-locking/src/lib.rs b/pox-locking/src/lib.rs
index b195f4cc9b7..05303a4d9b5 100644
--- a/pox-locking/src/lib.rs
+++ b/pox-locking/src/lib.rs
@@ -38,6 +38,7 @@ mod events;
 mod pox_1;
 mod pox_2;
 mod pox_3;
+mod pox_4;
 
 #[derive(Debug)]
 pub enum LockingError {
@@ -52,6 +53,7 @@ pub enum LockingError {
 pub const POX_1_NAME: &str = "pox";
 pub const POX_2_NAME: &str = "pox-2";
 pub const POX_3_NAME: &str = "pox-3";
+pub const POX_4_NAME: &str = "pox-4";
 
 /// Handle special cases of contract-calls -- namely, those into PoX that should lock up STX
 pub fn handle_contract_call_special_cases(
@@ -113,6 +115,15 @@ pub fn handle_contract_call_special_cases(
             args,
             result,
         );
+    } else if *contract_id == boot_code_id(POX_4_NAME, global_context.mainnet) {
+        return pox_4::handle_contract_call(
+            global_context,
+            sender,
+            contract_id,
+            function_name,
+            args,
+            result,
+        );
     }
 
     Ok(())
diff --git a/pox-locking/src/pox_4.rs b/pox-locking/src/pox_4.rs
new file mode 100644
index 00000000000..82bf38cdb1a
--- /dev/null
+++ b/pox-locking/src/pox_4.rs
@@ -0,0 +1,395 @@
+// Copyright (C) 2013-2020 Blockstack PBC, a public benefit corporation
+// Copyright (C) 2020-2023 Stacks Open Internet Foundation
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+use clarity::boot_util::boot_code_id;
+use clarity::vm::contexts::GlobalContext;
+use clarity::vm::costs::cost_functions::ClarityCostFunction;
+use clarity::vm::costs::runtime_cost;
+use clarity::vm::database::{ClarityDatabase, STXBalance};
+use clarity::vm::errors::{Error as ClarityError, RuntimeErrorType};
+use clarity::vm::events::{STXEventType, STXLockEventData, StacksTransactionEvent};
+use clarity::vm::types::{PrincipalData, QualifiedContractIdentifier};
+use clarity::vm::{Environment, Value};
+use slog::{slog_debug, slog_error};
+use stacks_common::{debug, error};
+
+use crate::events::synthesize_pox_2_or_3_event_info;
+// Note: PoX-4 uses the same contract-call result parsing routines as PoX-2
+use crate::pox_2::{parse_pox_extend_result, parse_pox_increase, parse_pox_stacking_result};
+use crate::{LockingError, POX_4_NAME};
+
+/////////////////////// PoX-4 /////////////////////////////////
+
+/// Lock up STX for PoX for a time.  Does NOT touch the account nonce.
+pub fn pox_lock_v4(
+    db: &mut ClarityDatabase,
+    principal: &PrincipalData,
+    lock_amount: u128,
+    unlock_burn_height: u64,
+) -> Result<(), LockingError> {
+    assert!(unlock_burn_height > 0);
+    assert!(lock_amount > 0);
+
+    let mut snapshot = db.get_stx_balance_snapshot(principal);
+
+    if snapshot.has_locked_tokens() {
+        return Err(LockingError::PoxAlreadyLocked);
+    }
+    if !snapshot.can_transfer(lock_amount) {
+        return Err(LockingError::PoxInsufficientBalance);
+    }
+    snapshot.lock_tokens_v4(lock_amount, unlock_burn_height);
+
+    debug!(
+        "PoX v4 lock applied";
+        "pox_locked_ustx" => snapshot.balance().amount_locked(),
+        "available_ustx" => snapshot.balance().amount_unlocked(),
+        "unlock_burn_height" => unlock_burn_height,
+        "account" => %principal,
+    );
+
+    snapshot.save();
+    Ok(())
+}
+
+/// Extend a STX lock up for PoX for a time.  Does NOT touch the account nonce.
+/// Returns Ok(lock_amount) when successful
+///
+/// # Errors
+/// - Returns Error::PoxExtendNotLocked if this function was called on an account
+///     which isn't locked. This *should* have been checked by the PoX v4 contract,
+///     so this should surface in a panic.
+pub fn pox_lock_extend_v4(
+    db: &mut ClarityDatabase,
+    principal: &PrincipalData,
+    unlock_burn_height: u64,
+) -> Result<u128, LockingError> {
+    assert!(unlock_burn_height > 0);
+
+    let mut snapshot = db.get_stx_balance_snapshot(principal);
+
+    if !snapshot.has_locked_tokens() {
+        return Err(LockingError::PoxExtendNotLocked);
+    }
+
+    snapshot.extend_lock_v4(unlock_burn_height);
+
+    let amount_locked = snapshot.balance().amount_locked();
+
+    debug!(
+        "PoX v4 lock applied";
+        "pox_locked_ustx" => amount_locked,
+        "available_ustx" => snapshot.balance().amount_unlocked(),
+        "unlock_burn_height" => unlock_burn_height,
+        "account" => %principal,
+    );
+
+    snapshot.save();
+    Ok(amount_locked)
+}
+
+/// Increase a STX lock up for PoX-4.  Does NOT touch the account nonce.
+/// Returns Ok( account snapshot ) when successful
+///
+/// # Errors
+/// - Returns Error::PoxExtendNotLocked if this function was called on an account
+///     which isn't locked. This *should* have been checked by the PoX v4 contract,
+///     so this should surface in a panic.
+pub fn pox_lock_increase_v4(
+    db: &mut ClarityDatabase,
+    principal: &PrincipalData,
+    new_total_locked: u128,
+) -> Result<STXBalance, LockingError> {
+    assert!(new_total_locked > 0);
+
+    let mut snapshot = db.get_stx_balance_snapshot(principal);
+
+    if !snapshot.has_locked_tokens() {
+        return Err(LockingError::PoxExtendNotLocked);
+    }
+
+    let bal = snapshot.canonical_balance_repr();
+    let total_amount = bal
+        .amount_unlocked()
+        .checked_add(bal.amount_locked())
+        .expect("STX balance overflowed u128");
+    if total_amount < new_total_locked {
+        return Err(LockingError::PoxInsufficientBalance);
+    }
+
+    if bal.amount_locked() > new_total_locked {
+        return Err(LockingError::PoxInvalidIncrease);
+    }
+
+    snapshot.increase_lock_v4(new_total_locked);
+
+    let out_balance = snapshot.canonical_balance_repr();
+
+    debug!(
+        "PoX v4 lock increased";
+        "pox_locked_ustx" => out_balance.amount_locked(),
+        "available_ustx" => out_balance.amount_unlocked(),
+        "unlock_burn_height" => out_balance.unlock_height(),
+        "account" => %principal,
+    );
+
+    snapshot.save();
+    Ok(out_balance)
+}
+
+/////////////// PoX-4 //////////////////////////////////////////
+
+/// Handle responses from stack-stx and delegate-stack-stx in pox-4 -- functions that *lock up* STX
+fn handle_stack_lockup_pox_v4(
+    global_context: &mut GlobalContext,
+    function_name: &str,
+    value: &Value,
+) -> Result<Option<StacksTransactionEvent>, ClarityError> {
+    debug!(
+        "Handle special-case contract-call to {:?} {function_name} (which returned {value:?})",
+        boot_code_id(POX_4_NAME, global_context.mainnet)
+    );
+    // applying a pox lock at this point is equivalent to evaluating a transfer
+    runtime_cost(
+        ClarityCostFunction::StxTransfer,
+        &mut global_context.cost_track,
+        1,
+    )?;
+
+    let (stacker, locked_amount, unlock_height) = match parse_pox_stacking_result(value) {
+        Ok(x) => x,
+        Err(_) => {
+            // nothing to do -- the function failed
+            return Ok(None);
+        }
+    };
+
+    match pox_lock_v4(
+        &mut global_context.database,
+        &stacker,
+        locked_amount,
+        unlock_height,
+    ) {
+        Ok(_) => {
+            let event =
+                StacksTransactionEvent::STXEvent(STXEventType::STXLockEvent(STXLockEventData {
+                    locked_amount,
+                    unlock_height,
+                    locked_address: stacker,
+                    contract_identifier: boot_code_id(POX_4_NAME, global_context.mainnet),
+                }));
+            Ok(Some(event))
+        }
+        Err(LockingError::DefunctPoxContract) => Err(ClarityError::Runtime(
+            RuntimeErrorType::DefunctPoxContract,
+            None,
+        )),
+        Err(LockingError::PoxAlreadyLocked) => {
+            // the caller tried to lock tokens into multiple pox contracts
+            Err(ClarityError::Runtime(
+                RuntimeErrorType::PoxAlreadyLocked,
+                None,
+            ))
+        }
+        Err(e) => {
+            panic!(
+                "FATAL: failed to lock {locked_amount} from {stacker} until {unlock_height}: '{e:?}'"
+            );
+        }
+    }
+}
+
+/// Handle responses from stack-extend and delegate-stack-extend in pox-4 -- functions that *extend
+/// already-locked* STX.
+fn handle_stack_lockup_extension_pox_v4(
+    global_context: &mut GlobalContext,
+    function_name: &str,
+    value: &Value,
+) -> Result<Option<StacksTransactionEvent>, ClarityError> {
+    // in this branch case, the PoX-4 contract has stored the extension information
+    //  and performed the extension checks. Now, the VM needs to update the account locks
+    //  (because the locks cannot be applied directly from the Clarity code itself)
+    // applying a pox lock at this point is equivalent to evaluating a transfer
+    debug!(
+        "Handle special-case contract-call to {:?} {function_name} (which returned {value:?})",
+        boot_code_id("pox-4", global_context.mainnet),
+    );
+
+    runtime_cost(
+        ClarityCostFunction::StxTransfer,
+        &mut global_context.cost_track,
+        1,
+    )?;
+
+    let (stacker, unlock_height) = match parse_pox_extend_result(value) {
+        Ok(x) => x,
+        Err(_) => {
+            // The stack-extend function returned an error: we do not need to apply a lock
+            //  in this case, and can just return and let the normal VM codepath surface the
+            //  error response type.
+            return Ok(None);
+        }
+    };
+
+    match pox_lock_extend_v4(&mut global_context.database, &stacker, unlock_height) {
+        Ok(locked_amount) => {
+            let event =
+                StacksTransactionEvent::STXEvent(STXEventType::STXLockEvent(STXLockEventData {
+                    locked_amount,
+                    unlock_height,
+                    locked_address: stacker,
+                    contract_identifier: boot_code_id(POX_4_NAME, global_context.mainnet),
+                }));
+            Ok(Some(event))
+        }
+        Err(LockingError::DefunctPoxContract) => Err(ClarityError::Runtime(
+            RuntimeErrorType::DefunctPoxContract,
+            None,
+        )),
+        Err(e) => {
+            // Error results *other* than a DefunctPoxContract panic, because
+            //  those errors should have been caught by the PoX contract before
+            //  getting to this code path.
+            panic!("FATAL: failed to extend lock from {stacker} until {unlock_height}: '{e:?}'");
+        }
+    }
+}
+
+/// Handle responses from stack-increase and delegate-stack-increase in PoX-4 -- functions
+/// that *increase already-locked* STX amounts.
+fn handle_stack_lockup_increase_pox_v4(
+    global_context: &mut GlobalContext,
+    function_name: &str,
+    value: &Value,
+) -> Result<Option<StacksTransactionEvent>, ClarityError> {
+    // in this branch case, the PoX-4 contract has stored the increase information
+    //  and performed the increase checks. Now, the VM needs to update the account locks
+    //  (because the locks cannot be applied directly from the Clarity code itself)
+    // applying a pox lock at this point is equivalent to evaluating a transfer
+    debug!(
+        "Handle special-case contract-call";
+        "contract" => ?boot_code_id("pox-4", global_context.mainnet),
+        "function" => function_name,
+        "return-value" => %value,
+    );
+
+    runtime_cost(
+        ClarityCostFunction::StxTransfer,
+        &mut global_context.cost_track,
+        1,
+    )?;
+
+    let (stacker, total_locked) = match parse_pox_increase(value) {
+        Ok(x) => x,
+        Err(_) => {
+            // nothing to do -- function failed
+            return Ok(None);
+        }
+    };
+    match pox_lock_increase_v4(&mut global_context.database, &stacker, total_locked) {
+        Ok(new_balance) => {
+            let event =
+                StacksTransactionEvent::STXEvent(STXEventType::STXLockEvent(STXLockEventData {
+                    locked_amount: new_balance.amount_locked(),
+                    unlock_height: new_balance.unlock_height(),
+                    locked_address: stacker,
+                    contract_identifier: boot_code_id(POX_4_NAME, global_context.mainnet),
+                }));
+
+            Ok(Some(event))
+        }
+        Err(LockingError::DefunctPoxContract) => Err(ClarityError::Runtime(
+            RuntimeErrorType::DefunctPoxContract,
+            None,
+        )),
+        Err(e) => {
+            // Error results *other* than a DefunctPoxContract panic, because
+            //  those errors should have been caught by the PoX contract before
+            //  getting to this code path.
+            panic!("FATAL: failed to increase lock from {stacker}: '{e:?}'");
+        }
+    }
+}
+
+/// Handle special cases when calling into the PoX-4 API contract
+pub fn handle_contract_call(
+    global_context: &mut GlobalContext,
+    sender_opt: Option<&PrincipalData>,
+    contract_id: &QualifiedContractIdentifier,
+    function_name: &str,
+    args: &[Value],
+    value: &Value,
+) -> Result<(), ClarityError> {
+    // Generate a synthetic print event for all functions that alter stacking state
+    let print_event_opt = if let Value::Response(response) = value {
+        if response.committed {
+            // method succeeded.  Synthesize event info, but default to no event report if we fail
+            // for some reason.
+            // Failure to synthesize an event due to a bug is *NOT* an excuse to crash the whole
+            // network!  Event capture is not consensus-critical.
+            let event_info_opt = match synthesize_pox_2_or_3_event_info(
+                global_context,
+                contract_id,
+                sender_opt,
+                function_name,
+                args,
+            ) {
+                Ok(Some(event_info)) => Some(event_info),
+                Ok(None) => None,
+                Err(e) => {
+                    error!("Failed to synthesize PoX-4 event info: {e:?}");
+                    None
+                }
+            };
+            if let Some(event_info) = event_info_opt {
+                let event_response =
+                    Value::okay(event_info).expect("FATAL: failed to construct (ok event-info)");
+                let tx_event =
+                    Environment::construct_print_transaction_event(contract_id, &event_response);
+                Some(tx_event)
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    } else {
+        None
+    };
+
+    // Execute function specific logic to complete the lock-up
+    let lock_event_opt = if function_name == "stack-stx" || function_name == "delegate-stack-stx" {
+        handle_stack_lockup_pox_v4(global_context, function_name, value)?
+    } else if function_name == "stack-extend" || function_name == "delegate-stack-extend" {
+        handle_stack_lockup_extension_pox_v4(global_context, function_name, value)?
+    } else if function_name == "stack-increase" || function_name == "delegate-stack-increase" {
+        handle_stack_lockup_increase_pox_v4(global_context, function_name, value)?
+    } else {
+        None
+    };
+
+    // append the lockup event, so it looks as if the print event happened before the lock-up
+    if let Some(batch) = global_context.event_batches.last_mut() {
+        if let Some(print_event) = print_event_opt {
+            batch.events.push(print_event);
+        }
+        if let Some(lock_event) = lock_event_opt {
+            batch.events.push(lock_event);
+        }
+    }
+
+    Ok(())
+}