I'll give you my thoughts, but I'm still a noob so take them as you wish.

The hands you are scared of here are 34 and 44, and I would say that completing in the SB in a multiway pot then b/r postflop is pretty consistent with that for a guy with those stats.

On the other hand, his 4x pot shove looks a lot like a bet that doesn't want to be called. Makes me think it could be a pure bluff, or maybe a semi-bluff with something like 56 or two spades. Whether he makes a play like that here might depend on your image (and button's), which you haven't mentioned.