I think thief/gardens could be pretty great in 4p. Am I wrong?
I've never tried it, but I agree, it seems like it could be great.
The advantage of it compared to Workshop/Gardens: One action card increases your deck size by (probably) 2-3, instead of 1, and decreases your opponents' deck sizes.
The big disadvantage: Your Action does not itself give you more Thieves or Gardens. This is worrisome, particularly considering that you're dealing with a 4 card hand in any hand with Thief, and most of your Treasure is Copper, and you have Green cards. Missing $4 to actually get more Thieves and Gardens seems like a real possibility. I suspect that there's a much higher chance that you'll want to buy Silver with $3 than in any of the other notable Gardens variants.
EDIT: I assume it's not simulateable? Geronimoo, Thief won't keep Coppers in the simulator, will it?