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.
I suspect I'm experiencing the same behavior. I don't have a 4K monitor to confirm but I can say that I don't get any display on my FHD monitor (HDMI connected) during the post. Once the AMD video driver loads (Windows 10), the display works properly. If I want to see the post screen (and to get into BIOS setup etc.), I need to connect a monitor to the VGA port. Very annoying. Unless I'm missing something, this is a big oversight on AMD's part. Not everyone has a 4K monitor.
This probably is different but sound very similar to a situation I had before.
I had a Acer X399H Monitor. It has a higher resolution than 1920 x 1080 but under HDMI the highest Resolution supported by this monitor is 1920 x 1200. Under DisplayPort I was able to achieve the Maximum Resolution which was above 1920 x 1200.
That being said, When ever I booted up with this particular Monitor, I had no video (unable to see BIOS Menu) UNTIL it booted into Windows Desktop. I found out the only way to be able to Boot into BIOS and and have Video was to connect the Monitor to DP on my GPU Card. So I had two cables connected to my GPU card, DP and HDMI. The DP cable I had hanging in the rear of the monitor and just hooked it up whenever I needed to enter BIOS and then disconnect from the Monitor and just have the HDMI cable connected.
Later, I decided to upgrade my Monitor to a larger one and purchased a Acer K272HUL. To my surprised, I had video from the time BIOS booted up until it reached Windows Desktop using just the HDMI cable. Unfortunately, I could only get Maximum Resolution of 1920 x 1080 using HDMI. The Monitor doesn't support higher Resolutions in HDMI connections. With DisplayPort I was again able to achieve the Maximum Resolution on the monitor.
Ryzen processor with G prefix has radeon vega graphic processor meant for 4k future multi applications big screen users.There are much more Ryzen with out G prefix.
ROG STRIX B450-F gaming motherboard is design to work with AMD Ryzen with radeon vega graphic processor.
So if you already own ryzen 2400G , I suggest you try above mother board. Or use your current motherboard and get any graphic card with DP1.2. which ever cheaper. I assume you have 8gb ram.
Your cpu is good, your monitor is 4k but your motherboard not matching, no DP 1.2 graphic output.
If you want 4k resolution use DP 1.2 (display port 1.2) or HDMI 2.0. DP is design by computer world for computer geek free royalty. HDMI is design by entertainment world for entertainment display with manufacturer royalty. DP has click secure plug in. HDMI plug depend on contact resistant is insecure for any serious 4K. So just look at your graphic card or mother board output if you have DP 1.2 in place.