I just bought these centercaps off ebay, but now I'm thinking that they may not fit Gt4 wheels from Am. muscle. Anyone know?

They are from Mach 1's.

 

i am pretty sure the only center caps that will fit the ct-r (and probably the gt4) are the oem "ford gt" caps and the 05+ center caps.

i would imagine you could probably get them to fit somehow... idk. something like grinding off the back and epoxying it to the gt4 center cap? now i am just completely guessing.

 

If you do have to modify them and end up using some sort of epoxy or glue make sure it's not permanent as they need to come off for balancing/mounting. Maybe some silicone would work so you can get them off when you need to.