UUIDv4 may or may not use a cryptographically secure random number generator. Python's UUID library, for example, falls back to the insecure 'random' module. Given a handful of outputs, it's possible to predict future ones.
def uuid4():
"""Generate a random UUID."""
return UUID(bytes=os.urandom(16), version=4)
So there really shouldn't be anyone using that today, thankfully.