How correct was played the hand?

Never cold call a 3bet (at least in cash games 100bb deep). Preflop you can 4bet or fold your hand, I would just fold because at NL5 playing tight can't be a big mistake.

BTN is betting big on a K-high flop, which is pretty good for the 3bettor. Your hand has not much outs to improve, you should fold.

Thnx for feedback

When do you opt to flat call 3bets instead of 4 betting ?

As the SB when you aren't the initial raiser, I would say never.

With a medium strength hand the initial raiser can always 4-bet and put you in a bad spot.

When you have a premium, you will be playing out of position post flop and you want to make the stack to pot ratio as small as possible to make your position disadvantage smaller.