The shower I am building is 36x42 on the inside of the curb. On one wall the 4ft wide kerdi will go exactly to the point where the tile will end. On the other will can I hang the entire 4ft wide piece of kerdi board and skim coat it with mud, sand, and paint, or do I need to cut it to fit exactly to the edge of the tile?
Assuming that you have drywall next to the Kerdi board it doesn't really matter. The backer board should reach the outside of the wet area. After that it doesn't matter what is behind your tile - drywall/backer. I often just run the backer to the point where I don't have to cut it or to the closest stud. Actually running your kerdi board to the edge of the tile is what you should not do because that seam would be more prone to crack. On most of my installs I try to hide the backer to drywall seam behind tile.