If you don't trust the FSF, the GPLv3 actually offers another alternative: "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 you could say "GPLv2, GPLv3, or any later version approved by ...". Which then means that a future relicense to allow GPLv4 (after reviewing it to your satisfaction) would not require contacting every single contributor.
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If you don't trust the FSF, the GPLv3 actually offers another alternative: "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 you could say "GPLv2, GPLv3, or any later version approved by ...". Which then means that a future relicense to allow GPLv4 (after reviewing it to your satisfaction) would not require contacting every single contributor.