Just finished an IRL 4-player game. I'm no good at 4-player, and have lost much of my former skill from the Iso days anyway, so I wasn't too surprised to find I didn't win (came in last, in fact).
The Kingdom:
Shanty Town
Spice Merchant
Cartographer
Explorer
Festival
Inn
Altar
Nobles
Farmland
Prince
So we all went Spice Merchant; myself and another player got a second Spice Merchant at some point. The others all hit $6 pretty quickly and just went Gold from there (Festival on $5). I had trouble getting to $6, but when I did I went for Nobles. I guess my thought was that Cartographer/Festival/Nobles could form a good deck; with the Spice Merchant removing the Coppers and Shanty Town for action support. But, I over-trashed my money, and before too long I was drawing my whole deck (except for the VP that Cartographer discarded for me), but my whole deck was giving me like 4 money. Oops.
So I know clearly why I didn't win; but I don't know if it was my plan that was wrong or my execution. Could a Cartographer/Festival/Nobles engine work here? Obviously the tricky part is that everything is super-expensive... so I wonder if Altar is the right thing to get with your first $6; so that you can start collecting Festivals and Cartographers, while thinning out the Estates. Maybe even skip Spice Merchant if you plan to trash Coppers to Altar. But of course, even then, Altar can't pick up Nobles; the only thing that draws.
So is it just too expensive for an engine; is the BM that my opponent's played the right way to go? And if you are just going BM, is Spice Merchant even worth it? I'm guessing so; though you'll want to choose the money over the draw more often than not.