I have the Full HD beamer. Of course I have no comparison with the 4K, but I am extremely satisfied! Luckily I got it second-hand for 600. Check out the second-hand market ;)

I ordered both and decided on the 4K. I asked myself exactly the same question. But I think the resolution of the 4K is better on a normal wall (I'm not a professional). With FullHD, you can see the small pixels if you look very closely. It's not the end of the world, but if it's possible, I'd go for the 4K. I have to say that I am enormously satisfied. Since I have placed the beamer above the bed, the volume of the cooling was also important to me. When the sound is on, I can't hear anything.

In my opinion, 4K is only worthwhile in a darkened room and with a perfect screen. Quality (and luminosity) with FullHD is impeccable.