I got the game recently and I’m completely in love with it. At the moment I only have the core set and I’m dying to play 4 players. Can I simply buy an expansion to field the extra players or do I truly have to buy a second core set?

And when I do buy a delux expansion, which one is good to start with? I love that Dunwich is near Arkham, so it seems like a good place to go for continuing investigators. The Dunwich Horror is one of my favorite stories. BUT I’ve been in love with the atmosphere of creeping insanity behind Hastur-related stories ever since I read the Tatterdamalion adventure from CoC RPG.

Buy a second core set. There's too many basic neutral cards everyone should have access to.

As to which deluxe expansion, I really think that's your personal taste as to which story you want to play through. You will also want to purchase the six Mythos Packs that go along with whichever deluxe expansion you get so you have the entire campaign.

Dunwich is pretty standard cultists and monsters. Carcosa has a lot of mess with your head stuff so in-story you're not always sure what's real and what's not. It is a bit more complex as well concerning the different ways individual scenarios can play out. Read the blurbs for the campaigns and scenarios to help you make a decision.

Carcosa has one last mythos pack to drop before it's finished.

If your going for four players and you will be providing the entire card pool then I suggest a second core set followed by the Dunwich Deluxe. Then just get everything in release order.

Get a second core. Then get one of everything else.

I actually supply the card pool for four players and I have 4 cores and 1 of everything else. I like it as we can have more flexibility and each deck can take a full slate of neutral skills but I don't think its necessary. Plus with only 2 cores, each card is unique.

You can add a third and fourth player just from all the expansions that have been released, but it will take time. Getting a second core will allow for four players right away, but unless if you have printed out a lot of fan scenarios, you will be limited to Night of the Zealot.