I would fold pre because BTN made really big 3 bet. Calling doesn't seems that great to me, can be +EV but marginal spot.
4 bet don't like too, because you said BU is tight regular I would prefer some other hands to use as 4 bet bluff

You could choose to 3 bet the original isolation raise but calling is ok too. Once you face a 3 bet you have to call $8.50 to win what's already in the middle $32.25 (13.50+13.50+5+0.25) + the remaining stack from ONE of the two players ($36.50) which equals $68.75. It's too optimistic in my opinion to assume that you'll stack both players in almost any situation.

We're getting 68.75-8.50 or close to 8-1 on our preflop call including implied odds and you're going to flop a set 12% of the time so you're roughly a 7.5-1 dog to hit one on the flop. This is likely going to be a really close decision and factors such as how splashy the recreational player is will matter a lot here.

Wow, thanks guys. So I was incredibly lucky coz I staked them both...
Fish cbet Aces, reg pushed flushdraw, and I had a set...

So low variance play would be to just fold pre to 3bet BU vs MP. I called bc in the vacuum fishes just call 3bets with garbage and I have nice implied vs tigh reg if I hit my set.

I just prefer to fold preflop
I hate cold calling 3bets generally, I could be doing it with JJ but that's all I think maybe sometimes TT/QQ.

As you called the 3b you can't fold on the cheap 4b.