9Ts vs BTN 3bet

Stats on CO and BTN?

If CO is a fish I would definitely call, if not it become closer, but depending on BTN's stats we might still be able to call. Definitely not a 4b though.

To me, it's a bad spot and a bad hand to 4bet bluff.

I'd simply fold here. Even if CO is a huge fish, it will be kind really tricky to play.

You don't have to change your game drastically. Make the changes one step at a time, as you will get in situations where you will not have the skills to make the most EV play or even turn a prefolp +EV play into postflop heavy -EV.
In your other hand you were defending JTo, now we see you defend T9s. And post flop it gets into a mess.
The good thing here is that you had a plan to bluff 4 bet and you strictly laid down your hand when faced aggresion.
Try to look at your ranges, see what you can call with when you face such wide 3 betting ranges. Then decide what you 4 bet bluff range will be. Choose hands with blockers, usually the first not fitting into the calling range.

This hand plays very well post flop, especially in multiway pots where we have great implied odds as long as we know how to play post-flop.

Yes, so we have to call, not 4 bet.