Hey ootm,1. First, since it is an unknown and have no reads on him, you can hardly assign to this specific opponent a 4-bet range yet. However, once you have a fairly good sample at a certain limit and are starting to have a feel for the players there, you can start to make some assumptions based on the pool of player you face at this limit. Then, you can assign them a 4-bet range and calculate the equity you have againt this specific range. Of course, it is not perfect since it can be different from opponent to opponent but vs an unknown, it is a good start. Normally a 4-bet is very strong; at the lower limit, I don't see a lot of players 4-betting as a bluff. Some people will 3-bet with a polarize range (very good or very bad) but in my opinion 4-bet is normally fairly strong.Suppose you have a hand and you are facing a 4-bet. Let's take the example you gave: "I have TT and villain 2,5x open raises in the 2nd blind level to 75. I 3bet to 225 and villain 4bets." Here you are a big favorite against all the small pocket pairs (66,77,88,99), are a big dog against big pocket pair (JJ,QQ,KK,AA) and you are flipping against (AK, AQ etc..). (this is a random range just for the purpose of the example). Moreover, at low/mid stakes, I think we can count on one hand the player that will laydown a hand they 4-bet with (even when deep) so we can disregard the fold equity. So, are you willing to make the shove?I see often a lot of good players/coach would based that purely on mathematic against unknown. In one way, they are probably right. Why not make a play that will make you money in the long run.However, they based that on the fact that losing a flip is not going to affect your futur play. Of course, some players are not subjected to tilt and are not going to flinch even if they lose some/a lot of flips in a row. On the contrary, there are players who are getting really annoyed when they lose 54-46, 52-48 flips and lose focus when it happens too often.So, my reasonning for this kind of hands is mostly based on your aversion to risk and how well you can deal with variance. Moreover, having an exceptionnal talent to play post flop (having a big edge against your opponent) would be another reason to consider laying down some hands.Example, I really like small-ball poker. If you watched cog dissonance videos, you can see him limping AJ, AQ, 99, TT and even JJ. A lot of people would say it is ridiculous to do that but having the kind of succes he has, it is clearly viable.So, in the end, I think it depends of your style, your aversion to risk, how good/bad can you deal with variance etc...I know I did not answer your question because I don't think there is really a clear answer.Anyway, I will finish with this:If you like the mathematical approach with a lot of stats, equity, EV stuff....I would advise you to watch all the Mersenneary (not sure I spelled it right) videos. Most are premium though since most of the stuff there is kinda advanced.If you are searching for a small-ball style with less variance, you can watch all the Cog Dissonance regular/turbos videos. (the ST are differents). Most of his videos are available with a standard membership.Always take my advice with a grain of salt.Cheers!