2k or 4k gaming

None of these. 1440p in this case. I dont recommend going to 4k with any setup expect for highend SLI/CF rigs. (Atleast two 980tis)
While a 980ti is pure overkill for normal 1080p, it would be perfect for 1440p gaming.
It will run any game just fine at max settings and it still looks ALOT better than just 1080p.
So i recommend taking a look at 1440p screens. :)

Im using a Dell u2711 for a couple of years now and wont ever go back to 1080p.
(27" 1440p 60Hz) Not really a "gaming" monitor but great colours. :)
 
if you can hold off for a bit read this:

"Seven new G-Sync monitors on the way, including 144 Hz IPS Asus display"
http://www.pcgamer.com/seven-new-g-sync-monitors-on-the-way-including-144-hz-ips-asus-display/

Particularly Asus PG279Q 165hz/IPS/27"/2k/G-Sync
Projected release is late this year or early next year at around $800.

I'm personally excited for the Predator X34.
 


Ignore the input ms rate, because every manufacturer tests differently. 5ms on manufacturer x is the same as 1ms on manufacturer y.
Just go for something between 1 and 8 id say or ignore it completely. Any new "gaming" monitors wont have higher ms rates than 5 anyway i guess.