I've noticed a lot of people having trouble with this, but I can't seem to find a solution. I've uploaded 4k video (which, btw, worked fine in an older version of Premiere) but it doesn't fit the screen. In the Program window there are black bars on both sides of the video, and when I render it at 3840x2160, there is a black box all the way around the video. Here are some screen shots: (I would greatly appreciate any help anyone can provide!)
Set Magnification to FIT instead of 100%.
Also make sure your sequence settings are the same as your footage.
Right click on the sequence in the Project window and go to Sequence Settings and check the frame size and adjust if necessary.
Ah, it turned out that I had to change the Pixel Aspect Ratio to Square Pixels It looks great now, thanks for your help