From someone who decided on a 1070 for 1440p:
Anything 1070 and up would be more than enough for 1080p for quite some time to come, so price is the only concern between the 1080 and Titan XP for that resolution. 4k will most certainly favor the Titan XP. However, since you won't be upgrading your monitor for some time, consider grabbing a 1070 and revisiting the GPU upgrade when that time comes. Reselling that 1070 in 6 months could recoup some of the cost. Lots of great advice in this thread, but if it were my decision to make, I would certainly want to see Vega and the (possible) 1080 Ti before dropping 1200 smackeroos on a GPU that I may not need.
I'll admit too that I haven't been keeping up with 4k offerings releasing this year, but when last I researched, you could get a 4k monitor with Freesync but not with GSync. After playing on a monitor with GSync, I'll never game again without one of the 2. Having an AMD card that can take advantage of Freesync on a 4k monitor running 60 FPS most of the time might be a pretty sweet gaming experience. Just food for thought.