what single gpu 300mm or less should I get for ultra 1080p up to 4k?

A GTX980 superclock would be appropriate.
Here is one, 10.51"
http://pcpartpicker.com/part/evga-video-card-04gp41982kr
IN particular, the blower cooler will be better for your case.
I might use a better quality psu than the corsair cx.
Seasonic, xfx, or antec in the 550w to 650w range.
A PSU longer than 140mm will need you to remove the hard drive cage.

I love those small cases;
I use a Q08.
Some thoughts:

Use a 2 x 4gb ram kit to get dual channel operation

You will want a better cooler than the stock cooler. Noctua makes some effective low profile downdraft coolers.
80mm is your max.

I might suggest using a 240-500gb ssd instead and omitting the hard drive.
You can always add one later. If you need to, a 2.5" laptop drive can be added later. That may be necessary if a 3.5" conventional hard drive will not fit. It will make your case easier to carry.