The migration guide describes the breaking 4.0 uuidRepresentation change (PYTHON-2245) like this:

The default uuid_representation for CodecOptions, JSONOptions, and MongoClient has been changed from bson.binary.UuidRepresentation.PYTHON_LEGACY to bson.binary.UuidRepresentation.UNSPECIFIED. Attempting to encode a uuid.UUID instance to BSON or JSON now produces an error by default. See Handling UUID Data for details.

https://pymongo.readthedocs.io/en/stable/migrate-to-pymongo4.html#encoding-a-uuid-raises-an-error-by-default

We should add guidance for how applications should decide to set uuidRepresentation when upgrading. Something like:
bq Applications that already store UUIDs in MongoDB using the PyMongo 3 default behavior need to set uuidRepresentation=pythonLegacy in the connection string. If they don't store UUIDs then they should set uuidRepresentation=standard.

We should also include an example error:

ValueError: cannot encode native uuid.UUID with UuidRepresentation.UNSPECIFIED. UUIDs can be manually converted to bson.Binary instances using bson.Binary.from_uuid() or a different UuidRepresentation can be configured. See the documentation for UuidRepresentation for more information.