In order of publication, they are:
In chronological order, the first of the novels is The Spellcoats, which recounts events in Dalemark's prehistory, when even the mountains and rivers were in different places. The rest of the Quartet takes place shortly before the [Great Uprising]?, in this order: Cart and Cwidder, Drowned Ammet, (the Spellcoats frame story,) and The Crown of Dalemark.
The first three books may be read in any order, but The Crown of Dalemark should be read the last of the four; it has characters from all three preceding books (even The Spellcoats: see [The Undying]?), and gives away plot details about its predecessors. If your edition of any of the books contains "A Guide to Dalemark", read that last, after even The Crown of Dalemark. It contains mahoosive and insidious spoilers for all four books.
There is also a novella set in Dalemark. Everard's Ride (1995) is not tied into the storyline of the Quartet, and can be read at any point in the proceedings.
Question: I thought the novella was The True State Of Affairs? [ACC03C4F.ipt.aol.com]