Im a beginner in PLO, this is a MTT.
Just wanna make sure my play is right.
Do we defend this on the button vs a decent regular who is a bit aggro spew and often shows up with weak holdings?

ahaha, well he is a profitable russian but does a lot of weird stuff.
Have you seen Veaaa playing for example, very good player but does a lot of weird stuff that some good regs would consider awful.

I don't see how you can consider folding this.

Flat versus 4bet shove? Im 20bb deep

Flat.

+1

Can you elaborate?

I only play cash omaha and have been out of the game for a while (just getting back recently) but I'm certain 4b shoving there is better than flatting. Can only assume that it being a tournament makes it different, but I can't figure out why

We're not a favorite against a 3b range, even a relatively wide one, but can gain a bit more EV from folding really bad flops (not just stuff like 987 monotone, but also 974r with no bdfd, etc.), whereas BB has to be less selective about stack-offs.

Our hand isn't one that exactly smashes that many boards though; when we GII on a board we like - such as the one that happened, we're often only going to be a small favourite. However, a lot of it's equity comes from seeing all 5 cards and whilst we might gain ev on folding really bad flops I'd argue we also can make mistakes by incorrectly folding on boards such as the 974r one that you suggested; it's the type of board that our opponent who is going to be 3b a hand like AQxx where xx doesn't connect with the board but prob has a lot of gutshot draws that jam it in; then we'd be making a decent sized mistake by folding.

The same can be true of board pair flops. Against a slightly spewy aggro villain capable of having a wide range we should be GII on most board pair flop but it doesn't feel that great doing so at the same time.

Finally, if villain is a regular to Hero then by shoving pre-flop with a hand like this every once in a while it's going to make villain re-consider 3betting Hero so light which should definitely be a good thing for us.