Also running GP4 on Win7 64 with no issues.
Did not installed at all...
Just copied all installed mods from my old PC running WinXP-SP3 to the new one.
Then I deleted files:
cards.txt
f1graphics.cfg
f1graphics.bak
... to force a new calibration of graphics and recreation of files in relation with the new graphics card.
Finally in each mod I set compatibility of GPxPatch.exe and GP4.exe to "Windows XP (Service Pack 3)" and enabled 'Run this program as administrator'.
... and all mods run fine, some of them with all 4 cores!!!
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some of my GP4 works ...