Wait, does the RPI do 4k? I did not figure it did at all. I would not expect to game on a RPI, even at HD, but if it can reliably display GUI stuff at 4k that is actually very good.
It can only do 4K at 15 Hz, which would be horrible to actually look at . Realistically, it can't do much better than 1080p.
But, yeah, yeah... it's only $35. Until you add the MicroSD card, micro USB cable, and (2+ Amp) USB power adapter. Now it's more like $60, and you'll still need a keyboard, mouse, HDMI cable, and monitor to set it up. If you need all of that stuff as well, you might be better off getting a used laptop for most use cases you would use a Pi for.
I used to use my Pi as a small Linux server, until I realized that I could get a more powerful server hosted in "the cloud" from a hosting provider like AWS or Google Cloud for free. Even a "micro" class server instance has more processing power than a Pi does.
Besides... If you want 4K for something like Netflix or digital signage, get a NUC instead.