I've never compared any 4K vs. blu on my system, so I don't know for sure. I'm guessing if there are any variations it might be in color or brightness. It would be a bigger issue with catalog titles that may have been remastered for 4K, but they include an older mastered blu-ray inside the same...
Yes. I play 4K discs on my non-4K displays (including projector) all the time. They look great.
You cannot play 4K discs without a 4K player, but the player can show them on any display whether it's 4K capable or not, limited to the resolution of the display.