Wanted to put this back in front of people; this is the only way we play 4 player Dominion now in my group, and I like it a lot. Some other clarifications we've decided on:
Any time a card says it affects "other players" (Embassy, Vault, etc), it means "opponents". Except Masquerade, because Masquerade is more fun if everyone gets to do it.
Attacks still don't affect your partner, even if you want them to (Spy and Oracle mostly).
Smugglers says "Gain a copy of a card costing up to $6 that the player to your left gained on his partner's last turn."
When you're gaining a card from your partner, it goes to the "correct" place. Nomad Camp goes on top of your partner's deck, Mine gains the treasure to their hand, etc.
Buying events is just like a regular game (If you buy Lost Arts, move your token, not your partner's), but if the event gains cards, your partner gains them as normal.
I'm looking at running a tournament for this in January; are there any other edge cases that there should be a rule for, one way or the other?