@Bog I don't know with regards to 4k, since I don't own 4k display/TV, so I'll let someone else chime in (although if my understanding is correct, you can't play 4k through computers just yet.. but you should be able to with 4k TVs)
As a Final Fantasy fan, I just can't get into FFXV. Tried starting the game 3 separate times, always got bored around the 10 hour mark. AC:O was a blast though. Played about 20 hours during Project Stream test before they closed the test (was given a code to be redeemed with Uplay, but it was choppy on my PC) and still hold my interest.
Definetly AC Odyssey. Great game.
FFXV is the worst final fantasy game ever in my opinion, highly overrated and nothing like any other FF game.
Both