v4 remove node14 support and removes many previously deprecated features
Use Messages.loadMessages instead of Messages.load. You don't have to specify the messages individually, and the eslint rules check your messages more effectively.
Messages.createErrorButPreserveName is removed.
Recent versions of v3 has SfdxConfigAggregator exactly matching ConfigAggregator. Use ConfigAggregator.
Was a copy of SfError, now removed. Use SfError.
Both of these were empty wrappers around SfProject and SfProjectJson. Use those instead.
Since this was moved to jsforce2, the sfdx-core implementation was an empty wrapper. Use jsforce's ConnectionDeployRecentValidation instead.
There are some deprecated methods (set, unset). They still exist, but you should use the newer async equivalent. They not only set/unset the value in memory, but immediately write to the filesystem.
write on aliases is now deprecated and a no-op.
AliasAccessor no longer inherits from the entire configFile family of classes, so those methods are no longer available.
This new version introduces file locks so parallel operations that write to Aliases should not cross-save each other.
These were independent functions on an object. Import the individual functions you need instead.
Ex: sfdc.trimTo15() should be trimTo15()