I disagree on that point - there was a 6-year gap between 4 and 5, the same amount of time between 3 and 4. The problem was the van hitting him, which A) changed the story tremendously, especially in DT7, and B) changed *him* too much to finish the story the way it would have finished before the "incident."
However, like I said, I enjoyed all of them, though I do agree on the weak ending - while the rest of that book was very good, the resolutions of major plot threads were just ... weak.