Would need a bit more context too help diagnose.
First question is if you’re HDMI or DisplayPort cable is actually plugged into the GPU or if it’s plugged into the monitor.
Now depending on your desktop environment (DE), you should be able to verify what aspect ratio your display is actually set to by going to your display settings. Again this depends on your DE, how to do this in gnome is different than how to do it in kde.
You can also verify if your game is set to output 2160p (4k). Sometimes game settings get reset depending on the game. Also make sure the game is actually using your GPU. Maybe try a linux native game like super tux cart just to help diagnose the issue.
Also how did you verify that you have the latest drivers for your gpu
I don’t have much experience with AMD gpus on Arch.
I was looking at the Arch AMD GPU wiki page and it looks like you might also need to run
pacman -S vulkan-radeon lib32-vulkan-radeon mesa lib32-mesa xf86-video-amdgpu
But admittedly there’s a lot about AMD GPU drivers I don’t understand. Like how to verify your driver in installed correctly.
You might also want to try out
amdgpu_top
orradeontop
to monitor what the GPU is doing.