This video is outdated but still helpful. My post linked below contains my updated list of the best reviewed Flicker Free 27-32" 4k Monitors.
Best Reviewed Flicker Free 37-32" 4k Monitor Post
Hardware.Info 4k Monitor Review Compilation
All 4k monitors are matte aside from the HP Spectre 32 which uses an almost-glossy coating (more information). The Acer XB321HK (Nvidia G-Sync; Toms Hardware Review), BenQ BL3201PH or BL3201PT (AMD Free-Sync), Samsung U32E850R (AMD Free-Sync) and Qnix UHD 325 are the best 32" options; the Acer and BenQ are the best. The Acer B326HK has excellent image quality but very high input lag according to both Hardware Info and Les Numeriques.
The Dell P2715Q and LG 27Ux8* monitors are the best 27" options; the LG is better for gaming since it supports Free-Sync (requires compatible AMD graphics card).
*The LG 27UD58 (no height adjustable stand or USB-C), LG 27UD68 (height adjustable stand, no USB-C), and LG 27UD88 (height adjustable stand and USB-C) likely all use the same panel and offer the same performance.