Playing 25NL with a player immediately to my left who seems to be 3-betting extremely light. I had barely any data on him, maybe 20 hands or so, but in those two rounds of the table I had seen him 3-bet from position maybe 3 or 4 times.

I pick up QQ in the CO, it's folded to me and I raise to 3BB, he 3-bets to ~8BB, the blinds fold and I 4-bet to ~23BB.

At this point he calls. This pretty much confirms the read and tells me that he doesn't really know what he's doing after the 3-bet. I cannot think of many situations where calling a 4-bet is the right play in this spot, since there are so few hands that don't hit the flop really hard that can continue and I simply don't have enough chips behind for that kind of speculation. Doing the math retrospectively, I see that this was far more marginal (although still true) than I realized if he was looking to spike a set and I should have been raising at least a couple of BB more to make the possibility that he was trying to do that greater +EV. If he was unpaired, this is a crazy play: the only thing that can continue is 2-pair or better and the odds against that would make it a crazy unprofitable call.

The flop comes KQ8 rainbow. I bet half the pot and he folds. Maybe too aggressive on my part, but there are so few hands he could have that I can extract value from here that I also need to think about the value of not revealing just how light I am 4-betting. If he's going to make a habit of calling 4-bets, I don't want him getting any ideas that he might be good after the flop with just top pair kings. In hindsight, the PF call was fast (just a couple of seconds), which means that it was probably a reflexive reaction on his part to facing a raise on the button with a PP. AA and KK shove here. AK either shoves or Hollywoods for 20s before folding. Probably nothing else calls, even from a really poor player who's not really thinking. Assuming that was the case, I should probably have tried to give him a price to see the turn and river, hoping he would spike a set for the rest of his stack. He'd have to be pretty unperceptive to fall into that trap anyway, though.

Sadly, I didn't have much time for this session and had to leave pretty soon afterwards. My question is: how loose would you start 4-betting this player assuming that he keeps three betting once every 6-8 hands or so? Assuming that I'm right about the PP, and his call range is basically 22-JJ and nothing else, how would you play him? How far do you think I could go before he would realize what I'm doing?

sorry but i wouldn't make an adjust by 20 hands of play. QQ 4bet into a happy 3better (if is really the case) is not a light 4bet.

if he calls 4bet in position, with a 4bet around 25~BB the spr must be a lot of times (100BB deep) < 3 only hands that are good to play EV+ in that situation are KK AA QQ.

If he 3-bets that much and its not just sample error then you can 4-bet for value AQo, 99+ . It would be very profitable to figure out if he 3-bets then gives up postflop or if he is aggressive postflop as well.

I think 20 hands is way too few to make any notes, except that V has a 3bet range. If he really is 3betting once every 6-8 hands, then he is 3betting almost 50% of all hands. That is very unrealistic. note ( I did not calculate this, but estimated rather )

Could as well be that his 3bet range is really narrow, but he got really lucky for few hands. Then he is being 4bet which never happens and does not know what to do. I think your assessment is right about him being an inexperienced player.

How far could you go with your line? Well it depends on what you mean. Firstly it is unlikely that you get into a position to 4bet again. So unless you start 4betting really lightly, then it does not matter since your line is just not happening again. If you are right and V is inexperienced, then he will likely overcompensate. Next time he might fold, but one after that you are facing a light 5-bet shove.

Then again, maybe he just meant to fold but misclicked call button.

And final point is that maybe you should just stick with your A game.