Why did you 3bet? There are a couple of reasons you would do this:
1. For value. Your hand is ahead of UTG's range, and you're 3betting to get value from that.
2. As a steal. You're expecting that you'll be able to take down the pot pf, or on the flop, often enough to justify 3betting here by representing a big hand.
Both of these generally require reads on UTG - if UTG opens an extremely wide range, JJ does pretty well against that range. If UTG plays fit of fold postflop, and calls 3bets pf, again we're in a pretty good situation.
In 25nl 6max, against an unknown, I don't mind flatting or 3betting - UTG ranges in 6max tend to be relatively wide (22+,AJs+,AQ+, KQs), so JJ doesn't fare badly (I don't think everyone will agree with me on this fwiw). In 25nl full ring, against an unknown, I'm not all that keen on 3betting, UTG ranges tend to be pretty tight.
Folding to the 4bet is good readless.