"4K" TV's are actually 3840 x 2160, which is four times the old 1920 x 1080 resolution.
A full 4K picture is 4096 x 2180. So you're loosing a strip down each edge rather than any height. 4096-3840 is 256 pixels or 128 per side, or about 6% of the screen width, 3% per side.
Some PC's will...