The answers are:
1) No. The base Mini will not allow you to comfortably view true 4K footage (it will display only up to 24 Hz, which is way too slow a refresh rate for a human eye for extended periods of time. Current movie productions tend to shift to high frame rates, like 60 Hz in "Hobbit" for example).
2) Whatever resolution you'd set in your YT player, there is no "spill" and would never be. You can set 4K and she player will scale down the stream to the actual display resolution. Set your res to 1280x720, select "4K" in YT and play it full-screen. See? It's the player scaling down (or up) the stream to always fit the real estate of the display.
1) No. The base Mini will not allow you to comfortably view true 4K footage (it will display only up to 24 Hz, which is way too slow a refresh rate for a human eye for extended periods of time. Current movie productions tend to shift to high frame rates, like 60 Hz in "Hobbit" for example).
2) Whatever resolution you'd set in your YT player, there is no "spill" and would never be. You can set 4K and she player will scale down the stream to the actual display resolution. Set your res to 1280x720, select "4K" in YT and play it full-screen. See? It's the player scaling down (or up) the stream to always fit the real estate of the display.