4K Footage comes in a variety of ratio’s.
1920X1080 is a 16:9 ratio. Therefore if you put footage into a sequence that has a different ratio you will see ‘black bars’ either at the top or bottom known as ‘letter box’ or on the sides know as ‘pillar box’. You get this even if you Scale Footage to the size of your sequence as Premiere doesn’t want to change the ratio and squeeze or stretch your image to fit the new ratio.
So what is generally best to do is bring in your 4K footage and scale it down using the ‘Effects Controls’ panel. As the ratio is different in your case you will loose a bit of the image on the left and right side but remove the letter box bars at the top and bottom.
If you want to avoid this in future, make sure you shoot at a 16:9 ratio (although having extra resolution on the sides can be useful if you later wish to reframe / tweak the shot.)
Hope that helps;)
John Williams
Soho Editors