I've heard the base set only gives you enough for two players. Does anyone know if The Dunwich Legacy provides enough for four players to play or would I still need to get a second base set?

I think I saw the DL deck lists use cards from the core set, so I'd say you'd still need a 2nd base set.

Hopefully somebody else can confirm this. My copy of DL isn't due until this afternoon.

I can confirm this. If you want to have enough for 4 players out of one collection of cards you will need a second core set. Everything from Dunwich on will have a play set of the cards from the expansion but to get the full play set you will need a second copy of the core set.