I have yet to find a mini PC that can actually output 4K via the HDMI port without sacrificing refresh rate. Unless you're okay with 30Hz SDR (which might be fine if you don't watch any HFR or HDR content), you'll need a PC with a modern GPU that has HDMI 2.1 support. Like a 3000 or 4000 series Nvidia GPU, for example.

But honestly it's cheaper to just buy a smart TV with Google/Android TV so you can sideload apps and watch whatever you want, without worrying about things like 4K120 and HDR working properly. If your current TV isn't smart, well now's a good time to sell it and upgrade to a nice OLED. November is one of the best times of the year to buy a TV at a good price.