Misc/NEWS.d/next/Library/2018-04-02-16-10-12.bpo-23403.KG7ADV.rst

``DEFAULT_PROTOCOL`` in :mod:`pickle` was bumped to 4. Protocol 4 is
described in :pep:`3154` and available since Python 3.4. It offers
better performance and smaller size compared to protocol 3 introduced
in Python 3.0.