who has centercaps from american muscle?

Are the Pony caps you purchased on eBay Genuine Ford (on the back they will have a small Ford logo molded in the plastic)? If so they will be the same as our caps.

The caps we sell that have Ford Logos (Cobra/FR/Pony/SVT) are all Genuine Ford Products, and as far as I've heard, we have had no issues with them fitting in our GT4 wheels. I believe the particular M-Fire caps that come with the GT4 are slightly domed, so any "flat-faced" cap might look slightly recessed. If you have any doubts I could test fit one here to double check.

One thing on your eBay caps to get them to fit is try wrapping the edge of the cap in Plumbers Tape, that should get them to stop spinning and stay tight.

He bought some OEM caps, because I believe the GT4's are like the FR500s, and the caps are about 1/2 has thick as the standard large size caps.