Gaming at 4k will depend on the game.
I seem to do perfectly fine with a GTX980ti at 4k for games like FSX skyrim and civ 4 & 5. I get a constant 60fps which is the limit of my monitor. (likely yours too).
But, for fast action games, a single graphics card, even a highly overclocked one, may not be enough. You should be looking to pascal and hopefully a much stronger single gpu card.
On the fan you linked, it is a 38mm thick one that I suspect will not fit in the slot.
Here is the strongest 25mm fan I could find:
http://www.frozencpu.com/products/7759/fan-471/Delta_120_x_25mm_Extreme_High-Speed_Fan_-_15033_CFM_FFB1212EH_Bare_Wire.html?tl=g36c15s60
Looking at your builds, I might suggest a blend.
Using the 901 case, use a i5-6600k and plan on overclocking it.
14nm runs cool, and a simple cryorig H7 air cooler will do the job.
The higher clock rate is needed to get full value out of a GTX980ti.
Use a Z170 motherboard to allow for overclocking.
As of 2/16/16
What percent can get an overclock at a somewhat sane 1.40v Vcore.
I5-6600K
4.9 11%
4.8 38%
4.7 70%
4.6 83%
On the graphics card, pick a EVGA GTX980ti, probably the ftw version which is the strongest.
If a stronger pascal card should be available within 90 days, evga has a free trade up program. read their web site for details.
Corsair CX is not considered as reliable.
And 500w is not sufficient.
Look to Seasonic for quality, and more like 620w.
On the fan, see how the stock 120mm intake does. You can always increase the speed later.