Could you open something like HWINFO64 in the background so you can see what is being pegged at 100%? It'll allow you to find where the bottleneck is. See if it's RAM (likely), CPU (likely), or GPU (maybe less likely?) .. or all three.
I think you might be better off buying one of those 4k Streamer Boxes.
My Nvidia Shield TV Pro has run every video file I've thrown at it.. just be sure to use Ethernet Cable for reliable playback of those high-bitrate files.
I used to have an Ivy Bridge / AMD 7950 HTPC, but the Nvidia Shield works considerably better and draws a fraction of the power.
Some old HW can handle it. I had issues running 4k streams with the old FX 8350 and RX480 and kept it at 1080p for a long time. But recent updates and I can run 4k HEVC and H265 fine now.