going from 1080p to 4k is significantly invovled more than 720 to 1080. upscalig is ALWAYS junk, fyi... some wil look better than others but you cannot put lipstick on a pig.
just like 4k tv;s... it's probably going to be a little fuzzy with upscaling.
also, it had nothign to do with the OS (win10)... it's all about your video card and software decoders. install MadVR with some ultra settings should do you nicely for video playback... vid card settings for how games look.
if you want larger text, you either need a lower resolution or increase text in the game... if the game doesn't support 4k natively, then the text you see is also upscaled lower res images, which can cause pixelation.
if you increase reso, text get's smaller => that's how it is supposed to work. it stays in scale... it's using the same number of dots to draw that character in a smaller space ... ie higher resolution.
you want a lower reso than 4k... (for what it's worth, every image you see on a 4k screen is 4k... even the 1080p signal from a blueray is displayed at 4k. the tv will scale all to its native resolution) so, even if you use "1080p" you are still seeing it in 4k
(the pc doesn't draw any more detail is best way to think of it and the tv is filling in the gaps automatically every single time you see an image take up the full screen of that monitor/tv)