The whole issue depends on how the 4-armed monster looks like precisely. Is that defined somewhere? E.g. I could very well imagine a person that has two front sides, i.e. two faces on each side and two arms on each side. This person could certainly fight with two greatswords without having problems with his arms disturbing each other. And the question of off hands and muscle strength in each arm (for the 1.5 x str bonus-issue) depends on the monster itself, too.
So how does the creature look like?