4K TV on Full HD Windows 10 and PS4 Pro?

I got a 4K TV a few days ago, but I read that if you play games (or media in general) on a 4K TV only with Full HD so that it is downscaled, it would look worse than native Full HD. For the context, I use my TV as a PC screen so that I can play on the PC even from a greater distance (from my sofa, which is about 3 to 4 meters away from the TV). Now my questions:

Corresponds to the facts described above the truth.
What if I leave Windows 10's resolution on Full HD? Does this deterioration effect then disappear if I also run the games on Full HD?
Does the deterioration effect catch the eye or is it noticeable at all?
If I leave the resolution of Windows 10 on Full HD, does Windows 10 also look worse than native Full HD because of the downscaling?
What about my PS4 Pro, is it also affected by the deterioration effect when I only play on Full HD?

1: Yes, as is generally the case with any flat screen, the image should be in the native resolution

2: no

3: Depends on the device

4: yes

5: Why should you run the Ps4 Pro which is designed for 4K devices in Full HD if you have a 4K TV that makes no sense

In addition, it should be said that it is upscaling and not downscaling, that would be the opposite!

Isn't downscaleing when a native resolution is screwed down and upscaleing when the opposite happens like supersampling? And I prefer to have more FPS than a higher resolution, so it makes sense.

The Ps4 Pro does not really have more FPS in Full HD (I use it myself)

Except at Rise of the Tombraider where you can choose and in return you do without more eye candy

You have nicer graphics for most games in Full HD Minimal it really makes more sense to run the Pro in 4K if you already have the corresponding TV