I've seen this on the top-left corner of the main panel! Now I know how to fix it
Great!
For what it's worth, I've done some testing on the 4x4s because I wasn't sure myself what the exact mechanics these days are.
I do know now...
When using a solid colour, you can (obviously) use the same 4x4 image five times, for border left/right, bottom and bottom corners.
But if you are using details (such as a line) in the borders, MusicBee will run some algorithm on the 4x4s that will slightly change the
colours and brightness.
So if you want pixel/colour perfect borders, you will need to use dedicated 4x1, 4x4 and 1x4 images.