Foucault said
Great spot to ask about. Against a nitty SB fold or call are both OK, but against a good player I think a smallish 4b is in order, your sizing is good. It’s OK to have a mergey range here because although you’ll sometimes fold the best hand to a 5b from AK, you’ll also sometimes get peeled by hands where having 99 is a lot better than having A4s or something (not that A4s would be a bad hand to do this with, just that you don’t want to be strictly nuts/air when you do it).

Flop is a perfect spot for a small (~25% pot) c-bet with your range. With such a low SPR, even nut hands like this one don’t need to make big bets to get stacks in by river, but you do want to bet something just to deny free cards to gutshots and backdoor flush draws. Likewise hands like QQ can get some value and protection with this sizing, A4s is happy to get some cheap fold equity, etc.  

Agree with all this. I think this is a really interesting spot because of villain’s likely merged SB 3-bet range – there’s some advantage to being poled here with our 4-bets if we can fold out a bunch of good equity hands, but also an increase likelihood of being peeled preflop instead of 5-bet.

If we do decide to have some cold-calling hands here, I think we have to size bigger with our 4-bets as our range gets more polarized. It seems like a choice between a strategy of merging our 4-bet range to include hands like 99, removing our flatting range and 4-betting to a sizing of around 19bb as we did, or adding some more cold-calls, polarizing our 4-betting range and 4-betting to more like 23-25bb. I think both strategies are valid, and the former might be slightly higher since it avoids the weird multiway 3-bet pot scenario where you’re stuck in the middle.