Why do you not instantly 4-bet him?.. because you know he holds a strong hand.
Why do you call a 3-bet OOP with a hand you already know is weaker than the range of hands he 3-bets?.. because you simply cannot accept that you would make more money folding than calling.. the answer is too simple and your mind is playing tricks with you. Fold!
When should you stop auto folding all but AA/KK vs a 3bet? .. when your opponent is betting a wider range than just JJ+ AK AQs. This happens when players strat trying to exploit you rather than bet their cards for value. When this happens and people start having 10% 3-bet ranges then you can play back at them with weaker hands or even complete air from time to time.
Until villians widen their 3-bet range you cannot play a 3bet/4bet/5bet game and this put you in the position you are in right now. When someone 3-bets you you have to fold almost everything and you cant accept that to be true. But until your opponents 3bet range widens, AA/KK are the only hands that really play well against it in 100BB pots! You cannot even profitably setmine in a 3bet pot with 100BB stacks.
Run another filter This time for pots where the pot is 3-bet preflop, you call and you are not last to act on the flop.... making or losing money?
Compare that to what happens in pots where you 3-bet, villian calls and you are last to act on the flop.
You know, I do hate it that you're right all the time

(kidding)
I am in the red calling 3-bets, I don't know how to filter PT to show when I'm not last to act, but nevertheless I am in the red almost 4 BI's calling 3-bets (I call 26% of the time it seems, so fold to 3-bets 74%)
The peculiar thing is had I folded everytime instead of calling I would be down more, losing just the initial standard 3.5xbb raise. But looking at the hands I guess that's just variation of the small 300 hand size with me calling 3-bets and hitting the flop more than the normal amount. And maybe villains' poor post-flop play.
I'd like to talk about the numbers though, you mentioned 10%, correct me if I'm wrong but I think 10% is 3-betting light some of the time, it means a range of 77+,A9s+,KTs+,QTs+,AJo+,KQo according to poker stove.
In real life I guess that probably means JJ+, AK o/s AQs for value and anything else such as small SC's, any PP etc. for 3-bet light for them. SO against a 10% 3-bettor I may be right to 4-bet JJ+, AK, AQs? At least OOP, and do some calls IP?
Most people are see are 5 and below, so a much tighter range where I guess I should fold anything but premium applying what you've said above.
What about AK OOP against that range 5% and below range though, that's been bothering me, should I still 4-bet with it, it has an
equity of 48.9% against a 5% range.
Ask yourself why is the villian 3-betting?
Is it because he holds a strong hand?
Is it because he thinks I will fold?
At micro-stakes it is almost always the first reason, he holds a very strong hand, he is not exploiting you, he simply holds a strong hand.
You're right of course, there are some people who do it though at 20NL. And I have a funny story to share for play at micro
I was 3-betting light in one game and this guy 4-bet me a few times, I did have a note on him that he 4-bets light...so one time he did it with me holding something less than premium (forgot what) I just called him. (I know, should have been shoving but was too chicken to do it then)
He got scared seeing that, hesitated, checked the flop, I checked behind, tried to
bluff at it on the turn, I re-bluffed and he folded.
So it does happen at micro, it is indeed too rare to worry about playing it...