Ultimate Reed was noted as having a different body structure. I also think 616 reed was noted as being somewhat different as well, but not to the extent of the Ultimate Reed. If this is the case, I'd imagine Reed living as long as Ben, but in Hickman's run we didn't see that. Now 616 Reed could have died from another cause, but I doubt Hickman thought the immortality would carry over to all four members. But that was a possible future, especially after Secret Wars' reconstruction of the multiverse.
Sue's powers differ from the others in that she affects other forces and items. The male three members of the FF had their physical bodies affected giving them powers from their bodies to do other stuff. The more i think about this, the more it bothers me, even though it's clear Stan and Jack didn't worry too much about this outside of making sure the powers were cool to read about and see.
But Johnny may very well be immortal while flamed on. I don't think it would be a stretch to assume he'd have the amount of time flamed on added to his natural life span. After all, he still has limits to how long he can remain flamed on, still, right?