> Hum, can you please write a short example of such class which can only be pickled by the protocol 4 currently? Just for my information.

For now there are no such classes in the stdlib. No one implements __getnewargs_ex__. But an alternative implementation of pickling for methodcaller could use it (I implemented methodcaller pickling in issue22955 in different way, via __reduce_ex__, but used the same trick for passing keyword arguments to constructor).

Note that multiprocessing uses default protocol 3 (issue23403), and this is not configurable.