The third one is long out of print and hard to find at times. The title is "First Flight." It's a 64 page book that I hear takes place mainly in the very distant past of Dinotopia on Poseidos back when they had technology, instead of being set during the 19th century like the first 2 are, though I think there are some references to Will reading the story as scroll. If you want to get it you may have to go to used book stores, or look at some places like Amazon or ebay to get a used copy, they don't have it even at the dinotopia.com store run by the Gurney family. I don't have it, only the first 2 and 4th.
The fourth is called "Dinotopia: Journey to Chandara." It's in format like the first 2, set in the 19th century on dinotopia with Arthur traveling to that city. I have the book but I've not read it yet, but have looked inside it at some of the paintings."
I have the miniseries, though some things are different then the James Gurney books.