No, you would never fold top set in a PLO game if that's the current nuts. An opponent can never bet enough money to make you fold, as your hand is never realistically going to be more than a 2-1 dog. In most cases, I'd just keep pouring money into the pot unless the stacks are very deep indeed. Once you've raised, been re-raised, and then popper him back again, it's pretty clear exactly what you both have. Unless he relishes a pissing contest, most likely he'll call and see what appears on the turn.
Collosal draws are very rare in PLO high and are almost invariably over-rated. The top set is a money favourite over all but the most immense draws, and is never a big underdog. To be a 55% favourite on the flop with a draw you need to hold a large wrap with a flush draw, and as that draw will rarely be to the nuts, it would be unusual for you to want to commit all your chips to the pot in a deep-stacked game.
For example, if you hold 589Tds in a deep-stacked game and the flop brings K67 with two of your suit, you'll be about 56% favourite to win if your opponent holds a dry set of kings, i.e. you're not a major favourite AND you want to reserve the right to pass against some opponents if a pair comes on the turn (he may give you a free card if not). However, in this spot, you can't be sure he doesn't hold the set AND a better flush draw, in which case you're looking at a not-too-tasty 34% pot equity situation and you definitely DON'T want to put your money all in. With this collosal drawing hand, on a PL table, I would raise a lead better on the flop but then just call if he pops me back for another raise (although I'd just put my stack in if it's a shallow stack game and hope for the best). As always in poker, you don't necessarily want to be making big bets when you are either a small money favourite or a large underdog.
Another reason for not going apeshit on your massive draws is that you'll often control a lot of the power in the hand by not doing so against a player with top set. For instance, on the turn vs a likely top set hand, with a draw that has about 21 or 22-odd outs; some would just crash into the pot and assume because they may be a slight money favourite that it's worth getting committed; quite often you'll find that 50% or so of your drawing outs (the hidden straight cards etc) will make your draw and STILL allow you to get a pot-sized bet out of your opponent (although he's liable to fold to obvious draws, like a flush or 3-cards to a straight on board) on the river. Therefore, you'll find that about half the cards you hit allow you to make your money ANYWAY by moving all in, BUT you don't lose the bet when the 45% or so of the blank cards that miss your draws (or pair the board) come. You may lose the chance to make the last bet if your big draw hits and your opponent doesn't pay, but you also SAVE the last bet if you miss (and you KNOW that of course your hand won't be good). The additional value of this move is, of course, you will get paid off a good number of the times that you HIT your draw, especially if it's a hidden or unusual straight, or a runner-runner flush. This is a pretty advanced concept and it's the stock good player's answer to the question "is it ever better to just call rather than go all-in on a shallow-stacked game when you're a slight money favourite on the turn?"
On the whole, draws are best played with CONTROLLED aggression. That is, when you have a truly COLLOSAL draw (or a merely good one when your opponent is showing signs of weakness, or is the sort to fold a marginal hand) you should be looking to bet out and even raise a lead better for the size of the pot. It is, however, a rare situation indeed when your draw is SO good that you want to pile racks of chips onto it before the end of the hand. That's not to say that SOMETIMES you shouldn't get all in on a TRULY monstrous draw in PLO by betting and raising, but usually in that spot your opponent has basically declared his top set by betting/raising multiple times, and it's often better if you're not sure you're well ahead to save some money to bet at the end. To expand on this point, it would be very very rare to make a RE-raise on the flop in PLO with a drawing hand - about the only time you might do this is if someone makes a weak-ish raise of your weak lead bet (so you're not re-raising full bets) or if the pot is three or four handed and you have something like a low wrap with a nut flush draw - say the flop brings 45T and I hold A678 with an ace-high flush draw, there's a bet, a raise, and a smooth call, I'd probably usually be looking to build this pot here; I might even make a minimum raise just to try to catch everybody along for the ride, but I'm certainly looking to play this hand for all the chips I can get into a multiway pot. As soon as it gets heads up with lots of action, however, I figure I'm up against top set and I can merely check-call along and save my bets in case I miss (up against top set here it's a virtual coin-toss as to who'll win by the river).
In the short-stacked games played online you should basically always fire all your chips into the pot with the nut set on the flop, raise until there's nothing left to bet. The times that you're a likely VERY small favourite against a draw will be more than balanced by the times you're up against someone holding something like middle set and a flush draw, or even a bare middle set (in which case you're a dominating favourite). The stacks are not deep enough for you to have to worry about keeping the pot small, and even in a deep stack game more often than not you'll want to just keep on betting against all but the tightest opponents - most people, as I said, over-value the big PLO draws and will happily let you re-raise them when they're holding a hand that's an underdog. You also want to get as much money in the pot as you can on the flop in case the turn pairs the board and kills your action on your NUT hand.
The only time we ever fold the current nuts in PLO high is when the hand in question is a straight, and even then it's fairly infrequent.
Anyways, apologies for the length of this answer but it's a good question and I felt I needed to have a good stab at it
Hope this has helped!
Monk
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