Here's one for you: The Martians is sort of a fourth book in this series. It's on my "to read" shelf also, so I can't tell you much about it (saving you from spoilers, and whatnot.), but I do know that it has earlier drafts of chapters from the first three novels, as well as short stories (at least one) continuing the story, and other information.
I found it disappointing. It seemed rather self-indulgent, a project that a less successful author would never have been able to get into print, because it was rather badly in need of editing and quality control, or at least clarification. The chapters jump among continuities rather randomly, and there's a sequence of non-contiguous chapters in a common continuity that initially seems like a prequel to the trilogy, but then veers off and turns out to be an alternate reality. It has some interesting parts -- I mainly keep it because of the chapter containing the full text of the Martian constitution set up in the trilogy -- but on the whole I find it a failure. It's certainly a letdown if you're expecting a continuation of the trilogy.