Once again, I must stress that the resolution is barely relevant compared simply to the screen size and viewing distance. 4K is meaningless after a certain distance, as 1080p will look equally sharp at that point. So too will 720p a bit further away, and so it goes. If this person is running 4K on a screen which is a couple of metres away, it'll very likely look the same if switched to 1080p.
Ergo, all that matters is that you run it at a suitable size compared to your distance from the screen... and I find that I'm rarely far enough away that I need to expand the window beyond 1:1. I lean into my PC to play with settings, which means that I'm less than a metre from the screen, and then it's all legible at 1:1.
In short: Make it the size you need it. If you can't read it, make it bigger. If you make it full size and the lack of sharpness makes it hard to read, make it smaller. Resolution is essentially irrelevant without more reference details.