A4s, 4bet spot, SBvsBB cbet Flop?

I wouldn't be 4betting unless you know your opponent is a reg. 4bet bluffing blind at NL5 isn't a good play.
You haven't shown any info on the opponent so I x/f is fine. You have no backdoor equity going for you and aren't getting any pairs to fold without multiple barrels, which isn't something you wana be doing vs unknowns.

BB is 18/16 (45) and looked like a reg, so I think he can be a reg

I think there are a lot better things to spend your time thinking about then a hand like this.
45 hands is not enough to get any sort of read from.
4bet folding a hand like this isn't bad and may even be profitable but with no info I prefer to just fold.
And don't worry about check-folding here, the profit comes from the pre-flop play.

Okay, thanks, I will reduce my 4bet bluffing range on microstakes. I wanted to try, because I saw it in some videos and I wanted to have experience with 4bet spots. But definitely I am going to focus on common spots.

I would fold to his 3 bet in the first place but...

At 2NL (for what its worth) my experience is when you 4 bet and villain does not put you 'all in' right a way c betting generates tons of folds.

The way I look at it is if you do a normal c-bet with air in say 5BB in a 6.5BB pot, that one is relatively more(!) expensive than c-betting 20BB in a 40BB pot.

You freak AK out for sure here and maybe even AA KK sometimes will find the fold button.
A tough cookie might call with JJ TT but they will be afraid also for AA KK.

Also it's easy playing later. You just give up when not folded

QQX boards always favor the call3b/call4b player, he has AQ/KQ/QJs/QTs and midpairs. Even with AA/KK you should be checking, you need to play your entire range way more passively.

@mbml If you check here do you not just give up?
I mean AA/KK has showdown value but with A2 you're always behind.
You think you rarely get a fold on a c-bet here?

I give up even with AK, you are basically dead vs typical call4bet ranges of QX and midpairs