Mostly I play 100NL 6max, and try to learn PLO at small stackes.
There was a hand that worried me a bit, if it were NL I would pass without a second thought.

I had in unraised pot in MP,

Flop (pot 1.25) ,
BB(full stack 25, as well as me) bets 0.5,folded to me I pot it, folded to him
he thinks and minraises.
At this point I was 90% sure he has quads, if one ever can be sure that someone has them, it was a felt about his timing, and how he tried to milk me, as well as a dry board.

Anyway we went all-in on the flop and he had 99.

As I just start to play PLO I saw a lot more quads, straight-flushes than in NL, so the question is if I should ever worry about the stuff like this? (Obvioulsy, I do not in NL).

I agree TG, I was thinking more of me being the aggressor, not so much on how I'd act if I was on the receiving end of the aggression.

Whether you want to go to the felt with JJ on a KJ6r board is completely dependent on your opponent and the game you're playing in. Probably it's correct at the lowest levels where people will jam their two pair or bottom set often enough, and from what I've observed in some bigger games the standard line for 100bb is to hammer it, apparently because you're good often enough to compensate for the unlucky times you're up against top set. Also, if you start folding those hands then you'll get run over.

An example from the $100PLO on stars, which I played yesterday: I raise pot with AAxx in LP, only one limper calls. Flop: A94, two diamonds. I pot, opponent calls. Turn: T. I pot, opponent checkraises allin. I'm ready to reload when I see a Qd fall on the river, but he flips over ATxx and he was drawing dead on the turn.
Now against this opponent I'm quite happy to play for 100bb with middle set on most any board...

However, in the average $50 PLO game against your standard TAG opponent folding is probably correct, and the deeper the money is the more correct it becomes.

There have been a couple times when I held top boat and ran into quads where I knew the other guy "had" to have quads due to his playing style---a characteristically rockish player isn't going to put a weird min-reraise in with bottom full, for example. I've had lines that went like yours, but I called on the flop, and then called additional milking bets on turn and river that put me all-in. In those instances, I think folding is the right play, although I admit I've never made that fold.

Fortunately, quads vs. top full just doesn't come up that often (at least in PLO8, which is what I play), so making this mistake from time to time isn't a big hit to the bottom line.

Of course, against even a marginally loose opponent, I think you can never fold top full if stacks aren't ridiculously deep.

There are just some hands where the poker gods are having fun at your expense and you are doomed to lose your entire stack - top full house vs. quads is one of those hands...

Not much you can do beyond telling them "nh", reload, and move on to the next hand.

My favorite top boat vs quads story is this one:

My 2nd or 3rd ever PLO8 multi, on Stars. I had been playing PLO8 for all of about a month maybe. I think it was the $20 buyin that runs (ran?) on Weds nights at 7:30est. Anyway, we were about 5 people from the bubble and I have been killing it. I am 2nd overall in chips and probably have double if not more than the #3 guy. Chipleader is at my table but he has me by maybe a couple K, if that.

I get QQ23 (or some other 2 low cards) on the button and it's checked around. Chipleader is in the BB. I raise pot and the SB and him call. Flop comes Q99. Check, check, I bet 1/2 pot. SB folds, BB hesitates and calls (clue #1). Turn is a 4, he pots. I figure he has to have 94 and I repot. He flat calls (clue #2). I start to get a little curious, as 9s full of 4s is pretty vulnerable and why just flat call? River is another 4, he pots. Ok, if he had 44 then I'm done. If he had 99 I'm done. The only hand I can beat is Q9, which at this point is no more or less likely than the other 2. I had 8 or 9k left IIRC and could have folded and at least made the cash, but I could not see laying it down with what I think was over 60k in the pot at that point. Of course he had 99.

I think I shot my dog after that hand. Pretty bad considering I didn't have a dog so I had to go to the pound and get one first, then take it home and shoot it.

Just curious, was the dog a black lab that responded to the name of Sparky?
He has been missing for 10 years and I would like some closure.

Tom

PS - I gladly pay off quads with a top boat. I also gladly pay off straight flushes with nut flush. I say gladly pay them off, because I hate when I have a monster that doesnt get paid and hate to see others have that same pain.