i7 3930k oc 3.9 ghz enough for gtx 1080 ti

Solution
It is plenty. For 4K the GPU is the bigger issue. Even with a GTX 1080Ti you won't be able to max every title out but you should get very solid performance. I run a i7 3930K @4.2ghz with 2 GTX 1080s for a 4K setup myself.
I suggest you drop the resolution to 2560x1440 where applicable (i.e. can't maintain 60FPS). Just set the monitor (on the monitor itself) to scale by ASPECT and it should work fine.

I've got a GTX1080 and can barely maintain a solid 60FPS in some games, and have to drop a few settings in other to do so. Plus, 4K often doesn't look noticeably better than 2560x1440 anyway; it can make a difference with aliasing such as the jagged edge "crawl" when horizontal lines aren't perfectly horizontal (in that case the higher resolution really helps a lot).

RESOLUTION is also just one factor. If you have to turn down other game visual settings just to maintain 4K it starts to defeat the point of 4K. There's always some COMBINATION of resolution and other settings that looks best to YOU at the ideal game SMOOTHNESS (FPS and frame time).

OC:
You can probably overclock higher than 3.9GHz too if you have a sufficient cooler, motherboard and power supply. I've seen it hit 4.8GHz though I don't like pushing processors that high.

(not every game is GPU bottlenecked even at 2560x1440 or 4K)

It's also silicon lottery, but if you investigate you MAY find that something like 4.3GHz is doable at a reasonable temperature.

In fact, if you haven't done so try seeing how high you can get the MULTIPLIERS for each core without modifying voltage. On my i7-3770K my multipliers (for TURBO values) in the BIOS where set for 1, 2, 3, and 4-core levels at x39, x38, x37, x36 or something similar.

I managed to raise mine to x45 so I hit 4.5GHz on all four cores which is a 25% overclock (on heavier loads which might drop to x36 at stock) with minimal change to temperature (since voltage is mostly what changes the power thus temperature).
 


Just remember, you may be "play on 4K" screen but doesn't "make it" 4K. You need to either enable in game or mod a game (like GTA V for example's 4K mod) to use Ultra High Poly models and true 4K textures to make it look 4K, not just 'upscale' the current 1K-2K textures to "fit" in a 4K display.

For example Andromeda currently requires even on a beefy system like that to drop to Med / High to play in 4K NATIVE (where the game includes these higher models and textures natively).