Can someone help me understand why I should be 4betting from the CO vs a BTN 3bet with hands like AJo/ATo/J10s/A5s, but flatting with hands like AQs/AJs/A10s/KQs/KJs/K10s?

It is a bit counter-intuitive to me.

One thing is if you get jammed on, it's much nicer to fold the offsuit hands. Also, offsuit hands are getting more of their value from making pairs, which is kind of naturally suited to lower spr play, and the offsuit hands still block villain's AA, KK, AK. On the other hand, 4b/f suited hands sucks when they are strong enough to flat the 3bet and retain suited playability. That said, plenty of suited hands are still 4betting co/btn.

funny, I was recently wondering about the exact reverse of this question - it used to be the case that competent players almost always used suited combos for passive actions and offsuits one for aggressive ones (especially for 3bets and 4bets), but with the advance of solvers, that is no longer the case, although there are still spots where that holds true, like the one you mentioned

the reason for favouring suited hands for passive plays was very simple and intuitive: suited hands are much easier to play postflop, as they give you some very nutted hands, as well as very natural semibluff candidates (so, basically what Duncelanas mentoned above)

however, the way poker is approached these days, people call a lot more often vs 3bets and, especially, 4bets, so raw equity advantage of suited hands means that they more often end up being played more aggresively