There are only 2 F4 secrets that I don't have. I don't have them on Vault, Mission 4 (where you fight 'Dragonman' on the elevators) and Doom, Mission 6 (where you fight Dr. Doom for the first time with Sue, Johnny and Reed on the rooftop). On Doom 6, the F4 secret is behind where you start (you should see it spinning around behind you during the cutscene). I don't knoww how to get it because it is unreachable. If you jump for it, you miss it and die.