One thing to note, hardware accelerated (Nvidia) is way faster depending on your CPU but they are not the same. It's not just using the GPU for processing power, its a different encoding engine. I have seen artifacts and strange stuff from time to time using the Nvidia encoder that I had to use CPU encoding to resolve (no pun intended)

What happens if some of my clips im editing are in 4k and some are in 1080p. Do i still just leave on 4k. If i already had dropped the 4k clips into thr time line when it was on 1080 if i change to 4k will it still change clips to 4k?