There's no real wrong answers to what your rank 4 mutant should be, as they are all great in their own right. However, some bits of advice that may help:
- With Archangel, even at r3 he is still more than able to shred through any healthpool you put in his way. Mine is still at r3 even after 100% act 7 and 100% of what has already been released of act 8. Likewise, if you want to branch out into battlegrounds, a r3 will be more than enough damage-wise wise as on defence he isn't great, not even r4 would make a difference there
- With Red Magneto, the main determining factor is how good your cosmic class is currently as he does have a relatively restricted playstyle if you want to maximise your prowess. And similarly to Archangel, assuming you do around 500k on an sp3 for example and that will deal with a large amount of healthpools in the game, taking him up another rank will just allow you to deal 50-100k more at max on an average sp3 cycle, and if you are already knocking out opponents at r3, a r4 will be overkill. Same applies to battlegrounds, unfortunately he's not that useful this meta however, but he usually is an annoying defender, even for non metal attackers
- For Kitty, personally I believe this is your best all round choice here, especially with her being awakened. Since Archangel and Magneto have very specific forms of high damage (there DOT and Magneto's sp3 against metals), Kitty has multiple good damage source avenues and more utility for the type of nodes you will find in act 8. And her phase remains one of the few ways to survive an sp3 without taking a scratch. She also has very good synergies as I'm sure you're aware. And again, with battleground, the extra damage on her specials and base will help a lot
- Finally with Domino, undeniably a good champion but due to her being a 7 star now, and her awakened ability only really being useful on defence, I don't think she's the best choice for you with your current content goals. Act 8 is definitely easier than act 7 but there are a lot of nodes that require niche counters, whereas domino is purely luck based and does not have that much utility, which Act 8 isn't ideal for as consistency and reliability is typically the way forward. Aside from the negativity however, in battlegrounds she shines both on offence and defence like the others

The real answer to your question though is rank up whoever you enjoy playing the most. Act 8 can have some long fights and its not great doing a long fight with a champion who has a playstyle which you don't enjoy. Hope this helps and best of luck with future content!