Playing CSGO 4:3 with an AMD RX 460 stretched its performance.
Try running 4:3 stretched with the RX 400 series driver version 17.6.1 to see if it improves your experience.
Select a 4:3 resolution option in the settings menu
Your image is displayed in a compressed format without scaling, resulting in black bars instead of proper resolution.
It seems like the display settings are on the front panel, not the GPU side.