I have encountered this scenario a couple times in the last month, once from the perspective of hero and once from the perspective of villain. Seeing it from villain's point of view has changed my mind slightly on possible courses of action.

Assume that hero is a tight aggressive player and that villain is a very aggressive player (i.e. capable of 3betting from the SB vs an UTG open with JJ+, AKo, a few light hands like A7s, etc). Also assume effective stacks of 130 BB.

If hero makes a standard raise to $12, gets one caller in MP, and the aggro SB raises to $48, do we almost always 4bet to $150 or is it ok to sometimes just call to under-rep our hand?

Last night i played as villain under slightly different circumstances. I had just sat down and UTG (young guy, had $400 in front of him in a 1/3 game) opened to $7. I looked down at AKo and 3! to $25 from the SB. He paused for a few seconds and just called. I felt his range was probably AK/AQ/AJ/AT/QQ/JJ/TT/99/88 and heavily discounted AA/KK. When the flop came QJ7r I fired a bullet to try and represent AA/KK and push him off AK/AJ/underpairs. I fired a second bullet on a blank turn (which in retrospect was pretty spewy; only hand I fold out that flop bet doesn't fold out is AJ I think), checked the river and he turned over KK. His flat call preflop has some risk in that both me and the MP player get to come along cheap, but it opened up my range to continue firing bets with hands that he beat on the flop. If he 4bets to $150 most players can easily fold AK and JJ and sometimes QQ, so it prevents some further gains (esp on flops that come ragged and dry).

Thoughts?

Flatting an aggro is how we make money. So yes, flatting utg is almost optimal - only if we know v to be aggressive and inclined to barrel.

I think you kind of answered this one yourself. We "almost always" 4bet. I think 4betting is the "standard" play because it gives you the most value. Against an opponent like the one you mention I would very rarely deviate from the 4bet. If villain is an uber-tight OMC then that's a different story. However, against an aggro opponent the 4bet will be so often the correct play that I think its too -EV to just mix it up for the sake of mixing it up.

The reason we 4bet is to build a huge pot with a premium hand preflop. So in order for the smooth call to be the better play we need the value gained from keeping his range wide to be greater than the value of his calling with worse. Obviously this will depend on the villains calling tendencies; but in this case I believe that we surrender value from hands like JJ-QQ/AK that will call our 4!.

Yes it is great spot to flat KK.

If I had to put this in one sentence of wisdom, it would be "every time we raise, we thin a villains range"

So if he were to 4! he would likely fold out QQ/AK which are hands he definitely shouldn't mind being up against.

In other words he is keeping your range as wide as it possibly can be by flatting.

The other key here is position. The 3!'ing sb has taken the initiative, and he is almost always cbetting his entire range. And often double barreling a wide part of his range as well.

So we are quite fine flatting the whole way and/or sometimes raising turn.

The times we don't want to flat are when villain is a station and will call with worse, or is aggro, and will jam with worse if we click small.

Another interesting tidbit is that the 3! came from the sb which is one of the "strongest" 3! positions you will see preflop. (i.e., when someone 3!'s from UTG+2, the sb, or the bb, it is generally a fairly strong hand)

Raising only "narrows" that strong hand range by folding out AK/QQ and keeping in KK/AA

The same idea applies to aggro villains but even more so bc they are firing so much of their wide range. When he 3!'s with A5o for example and we 4! KK and he folds, he makes the correct decision. By flatting, we allow him to fire into us post flop for one or two barrels.

I don't disagree with anything you said in your post, but I think this part in particular is a bit oversimplified. If villain is a TAGgy player who regularly 3bets light enough in this spot (which is saying a lot given that we're UTG) for us to be aware of and concerned with it, then you'll want to have 4bet bluffs in your range against him. And if you have 4b bluffs in your range, you need to be very careful about how many value hands you're flatting with, lest your 4b range become filled with hot air (though we're IP here, so I guess we could use a strategy of having a wide flatting range and no 4b range at all, hoping to raise a lot of flops or float often; OOP this wouldn't even be an option). And in the calculus of "is this player more likely to stack off pre or post?", players that are aggro pre and especially those with whom you have an aggro dynamic are less likely to fold stuff like JJ-QQ/AK to your 4b.

But yeah OP, I don't know if having a "standard" play here is really necessary or even preferable, given how villain and dynamic dependent this spot is. It's that "pre or post" calculus, along with being somewhat mindful your range construction, that should determine your move here. If the villain you describe is likely to fold AK/JJ to a 4b and sometimes folds QQ as well, then yes this is a turbo flat IP (OOP I'd probably still 4b, but I'm on record as hating playing OOP more than going to the dentist; it's pathological at this point), but lots of players can be pretty sticky those hands pre but more cautious postflop on unfavorable boards, in which case you'd prefer to 4b.

I never 4bet AA or KK if I know it's gonna be heads up vs an aggro or someone I know is barreling out no matterwhat.