If code is licensed strictly under a specific version of the GNU GPL, future versions have no effect for that specifically-licensed code. For code that is licensed under "version 3 or any later version," then recipients could choose to apply (a possibly more permissive) GPLv4, whenever one comes into existence, and enjoy whatever permissive terms that new version affords.
Indeed, this is not a pure hypothetical: the FSF has historically done something similar with the GFDLv1.3. (GFDLv1.3 allowed -- with careful time constraints -- wiki site operators to switch users' works under GFDL1.2-or-later over to the usually GFDL-incompatible CC BY-SA 3.0.) This case was done very carefully, but it does highlight that some action that is disallowed by one version of a GNU license may be allowed in a future version.
You could instead specify a proxy to authorize the use of future GPL versions. The relevant legal mechanism is detailed in section 14 of the GPLv3:
If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. [...].
If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.
So if there is another entity you trust to represent your interests better than the FSF, then you could say
... either version 3 of the License, or (at your option) any later version that is approved by [identity of proxy].