Well here's the thing, a while back I was planning on doing a town map and was trying to gather information on roughly what the buildings to population ratio would have been for the medieval period. Mind you, I didn't spend hours in the library researching academic sources (though, oddly, my inquiries did lead me to read portions of an old dissertation from the 50's), but in the few hours that I did devote to this research the best resource that I could come up with was the A Magical Medieval City Guide. This guide suggests that this ratio is approximately 1:2 (1 building per two adults). So if those figures are right (and I have no idea if they are or not), then you'd be looking at 100-200 buildings, depending on whether or not that 400 estimate includes the entire populace or merely its adults.

As an aside, I highly recommend this supplement. There is also a pretty nifty online City Generator that CyrstalBallSoft designed based on this supplement. Hope that helps.

Cheers,
-Arsheesh