GPLv4 cannot exist. The GPL was a clever hack the first time it became popular, but it is fundamentally a singleton that cannot be instantiated more than once.
Most of the time, when you license something under GPLv3, you explicitly allow for later versions of the GPL. The "either version 3 of the License, or (at your option) any later version." part was famously removed in the Linux kernel.
This clause is a bit dangerous -- if a bad actor gets control of the FSF and manages to release an official GPL version that is completely bonkers, all current GPL-or-later software can be distributed under that license.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This has nothing to do with copyleft outbound licensing.
If you have BSD-style outbound licensing and you assume that all inbound copyright is transferred to you, you will wake up in court after you try to change the outbound license conditions of someone else's copyrighted code.
The correct answer is that for any project in which there is no inbound contribution agreement in place, you would have to contact all contributors and seek their permission to relicense.
Not like we didn't try.