The bit depth has very little to do with colour representation on these monitors. Modern FRC dithering algorithms are very good at representing shades that can't physically displayed without dithering. Regardless of bit depth the colour accuracy suff...
The PG278Q doesn't have integrated resolution scaling capabilities. As a result, the GPU handles the scaling. And as far as I'm aware the graphics processor on the PS4 can't take a 2560 x 1440 input so it wouldn't work.