What Psychotrip said, pretty much 100% agree. Sure you can RP in the sense of playing the role they force upon you, but if you want to RP a different character the game hardly supports it at all. You can play different versions of the same character, while in 3 you could play different characters, they shared a vague backstory but they could vastly differ, in FO4 it's essentially impossible to have the PC not care about their family, in several instances even the O button option, which seems to be the "[censored] you/I don't care/blunt no/go away/violent/etc" button is just a louder complaint about having lost their baby.
During sidequests then ya, you can act like whatever, and sort of in the main factions quests (aside the parts about them supporting you for the central plot). Also, more subjectively, playing evil doesn't fit and barely seems to have any impact. In my 1st file I joined the BoS, then later betrayed them, and when I fought the MPC who recruited me face to face he didn't say [censored], didn't call me a traitor or question my apparent change of heart or anything. They knew my PC and when they were betrayed they just appeared as an elite mook with a name. In my 2nd/parallel file, I can eat corpses right in front of companions and they don't react at all. In fact cannibalism no longer has any drawbacks of any kind, people don't care when they see it, it heals more than most if not all food items and without any rads, it doesn't even have karma to at least have some sense of being morally grey. It's just a really good extra healing ability.
Also also, being evil hardly has any positive effects anymore either, killing civilians/settlers/etc doesn't give xp, you can't join any flat out evil group and/or make big flat out evil decisions and from what I've experienced so far way too many arbitrarily "essential" NPCs. If I need a key why can't I just kill the guy who has it ? Or why can't I kill any of the minutemen ? Preston I guess is fine cause he's their leader and gives all of that factions quests, but you can't kill any of the others either. No small evil decisions too afaik, in FO3 I could sell kids into slavery, in FO4 I can...talk rudely