I would have raised the river. SB's play is confusing but I think SB is slow playing an overpair. The play up to the river seems to say QQ KK or AA. If SB flopped a set wouldn't we have seen a check raise and not two check calls. I can see the post flop check call to keep people in but not the turn check call. A check raise on the turn is unlikely to take it down given its size and four players, so why not build the pot. My guess would be KJ or QJ and the set made him feel better about the top pair he had been nursing through the hand.

Although I would not have been able to work it out in real time. When I am confused I check the river.