It was easy to be fooled in in thinking some DC-4 had square windows. Quite a few small (and shadier) operators painted squares around the round windows of their DC-4 in an effort to deceive people into thinking it was a DC-6. They were pretty artistic efforts. The black rubber deicer boots on the wings and empennage of the DC-4 were of course a dead giveaway as the DC-6 had heated leading edges of dural.
By the way, "square" windows were not square. They had generous radius's in their corners eliminating the stress riser of a sharp "square" corner.