It'll almost certainly be up to the devs and if it is, I can guarantee you there will some games that will not run at 4K. That said, I do expect devs to target 4K rather than focusing on more important stuff, so in that sense, I suppose you are right. I'm not very optimistic about this...
Sure it is up to devs. But start of gen it is full of crossgen, so it those games are 1080 or close in PS4 and X1 they will be 4k on next gen.
Some devs will chose to go below 4k some time after, perhaps 1440p. But certainly they won't go 1080p even less for the reason of putting 60fps. If 60fps is the most important thing for you then PC is basically the only option.
1080p will probably be only used by indie devs. There is to big of a generational jump from base X1 to next gen for 1080p to be the general choice anyway.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"