@bonifartius No, you may not; "The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns."

Any possible "GPLv4" published by a Proprietary Software Foundation is not from the FSF, thus it would be invalid.

A weak license would not be "similar in spirit" and therefore would be invalid also.