Board Thread:Questions and Answers/@comment-26008058-20150208132004/@comment-34326521-20150210032852

It says "not full sized Humungousaur" as if it was a completely different being from regular sized Humungousaur. This was the first indication I got that the OP wasn't looking for an actual answer; just the one he/she wanted. If you remove Humungousaur's signature ability, you won't have a fair scenario, and your results (in this case, the question) will be meaningless. Even Grey Matter could pummel Way Big unconscious if Way Big isn't allowed to move, fight back, or heal (as all beings do, on a small or large scale).

In a fair scenario, Humungousaur is stronger than Four Arms. In a fair scenario, Four Arms is more agile and has four arms. In a fair fight, Four Arms would win. He has the upper hand on agility and number of arms. Even if Humungousaur's two arms weren't completely stopped by Four Arms's upper arms, his lower arms would easily let him win.