Who the **** told them to FLAT 4bets?!

Used to be shove or fold imo but nowadays seems like their flattin a lot(at least on my site), even oop..
Although these are mostly weaker regs ime, i still haven't figured Out the best way to exploit them for doing it.
As my cbet (which usually is about 1/3 pot in 4b-pots) doesn't work out a lot anymore, since they realize they can't Play fof everytime, i am often lost on the turn as i don't really know which Range to put them on,also because i feel they mix in slowplays w aa/kk too sometimes.
That being said i don't really feel like i'm 4bing TOO much really.
Any ideas? Which is the best Way to counterfeit in these situations?
I do realize this somewhat makes their 3b-Range more depolarized and we should be inclined 2 4b for value, barrel singleraised pots relentlessly as their flattin-range gets weaker and maybe attack alot of middle- and small-Card Flops in 3b pots, cause they are 3bing hands like kq, ajs,etc. But still i don't really enjoy callin 3bets oop and we can't only go for value. Can someone help me Out pls?