Can't really answer for the F-4, but it's generally accepted that the first racing car to use carbon fibre in its construction was the Lola T70 of 1965. So if it was in use with race car constructors by the mid-sixties, it's easily possible that Phantoms were being manufactured to include it ten years later. Production ended in 1979, so plenty of time to incorporate CFRP into the later models at the very least,