[pieno]

The risk of a coup is already covered to some extend in the GPL itself[0]. Article 14 which also governs the “or later” spec says: “Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.” It’s not crystal clear but you should be able to invoke this if GPLv4 would no longer be compatible with the basic idea of a “free, copyleft license” described in the preamble. You could then say that GPLv4 does not apply to anything licensed “GPLv3 or later” because GPLv4 is incompatible with that basic idea.

Also, any “relicensing” would only apply to changes made after the license change. You cannot unilaterally revoke the original “GPLv3 or later” license on works that are already released under that license (see the term provision in article 2 of the GPL).

[0] http://www.gnu.org/licenses/gpl-3.0.txt