FC4 and FC5 have some structural differences. I think a decent amount of feed back was taken for FC5. It doesn’t have tower climbing puzzles and the collectathoning which had gotten way out of hand in FC4 was toned down a lot in FC5. FC5 replaced some of that with the totally optional survivalist bunkers that give rewards per bunker. It’s hard to go back to FC4 and it’s “collect 300 scraps of paper” nonsense after FC5.