I'm getting the exact same issue with a Ryzen 3 2200G on an MSI B350M. No resolution yet from MSI. Checked MOBO out with the loaner A9 chip sent by AMD in the boot kit and it worked at 1080p. I'm fairly confident this is a CPU problem. I just don't know exactly why.
In Windows 10 Settings under "Display" do you have it set for 1920x1080 or is it set at "Recommended" which might be at 4k resolution since you have a 4k Monitor connected.
I don't believe the CPU has much to do with the Resolution output to a monitor. The GPU installed and its drivers is what determines the Resolution output on a monitor. Plus how the GPU software is configured for the resolution you want running.
I realize that both the CPU and GPU work together as explained in this comment at a Forum: Tom's Hardware Forum
What is your CPU? More stress on the GPU=more on the CPU.
Not really true there. Higher FPS does cause more CPU stress, but if the GPU stress comes from a high resolution, it can lower the CPU usage due to lower FPS. What the CPU does for the GPU isn't affected by the resolution.