There's a huge difference between DVD and Bluray quality even on a smaller screen, I've never bothered to upgrade because for the most part I don't care enough to spend the extra money for the gain at this point. I just want to watch the stuff. Very few people have the equipment to showcase 4K though, and you'd need a big screen to notice the benefit over 1080p. I think by the time 4K becomes standard, the physical formats will be mostly dead anyway.
I love all three BTTF movies too, but 3 has always been my least favourite (not a huge drop-off, just a little less than the perfection of the first two). I appreciate it more and more over time, but the setting and story don't appeal to me quite as much as the setting of the other movies. It's still a damn near perfect trilogy.