4-bet sizing is a interesting topic. I'm curious to hear other opinions. With 100bb stacks you have four options:

1) Never 4-bet. This doesn't mean you call with AA/KK/AK and fold everything else, b/c if villain is paying attention they'll 3-bet you to death. You have to call with premium, good and speculative hands. Pros: Deceptive. Decreases variance. Allows you to outplay your opponent postflop (pick off c-bets). Cons: You'll be playing big pots oop occasionally. You may lose value from your premiums.

2) 4-bet small. By small I mean less than pot-size, so from $12 to ~$28. Pros: This bet size makes it mathematically correct to fold light 4-bets to 5-bet shoves from your opponent, reducing variance. Cons: May not have as much fold equity as a larger 4-bet.

3) 4-bet larger. Pot-size would be from $12 to ~$37. Pros: Might have more fold equity than a smaller 4-bet. Cons: Makes it mathematically correct to call most 5-bet shoves, increasing variance.

4) 4-bet AI. Pros: Probably the most fold equity. Great for metagame, gains value for AA/KK. Cons: Light 4-bets are obv getting the worst of it when called, variance goes way up.

I'm still refining my 4-bet strategy, but I try to shift between these options depending on my opponent. The +EV of light 4-bets comes almost entirely fold equity and any difference in equity when called is a side benefit, so FE is what I'm usually thinking about.

As for 100nl, I've played very little 6max there, but my guess is that light 3-betting is very profitable and light 4-betting not so much.