could be lots of things, rich. If your interviewer is ill, that will delay your post-interview review. Or problems getting the adcom together for some reason or other. Or backlog of applications for the review committee post-interview (I hear at one point VCU was 4 weeks behind - this can happen if there is lots of discussion on certain applications chewing up lots of time).
If in doubt, call them and ask. Just say something like "they said it would be 4 weeks for letters to come out, and it's been 5... I just want to know if the letter got lost by the post office, or something." If you don't blame it on them, they're usually quite accomodating and forthcoming with information.