I don't really have ideas for the rest of the F4 to be honest and can't think of other actors at the moment either. I recall picking the cast for a potential live-action Jonny Quest movie over a decade ago and thinking how they could kind of match the F4 too, but can't remember what names I came up with. I think I had Daniel Craig as Race Bannon (so technically he would be Ben Grimm here, though I probably not the best choice for the role). This was around the time Craig started out as James Bond, I think.