Farcry with 3 970s in sli on ultra gives 40-45fps
COD with 3 970’s in sli on ultra gives 60fps
Those are two newer ones^^
A single 970 on games that only need a lower spec gpu (BF4, Fifa15 etc) should see you at about 40fps ish, maybe a tad less depending on settings.
The best way to find your fps is to google fps of a game at 1080p with a single 970 and then divide it by 4 (as 4k is just that 4x 1080p)
The 970s and 980s are crap for 4k to be honest, the R290x gives waaaay better performance at 4k on a per card basis due to bus width.
But if it’s only for BMS, then a 970 will be fine (290 is better) and a 4790 is overkill, as mentioned an i5 will do the job perfectly.