tjt35,
We seldom see used 4x20 series cab tractors down here. People seem to like them so much that they are very hard to find used. When I do see them, however, they are almost never advertised for less than 32k unless they have excessive hours or are in poor condition.
For example, the only thing in the region right now is a 2010 4720 (not a perfect match, but close) with roughly 100 hours, 4wd and fel for $37k. Unless your 4320 cab is in poor shape for 600 hours, I say you got a good deal. In fact, I would expect to pay closer to $30-32k for the same tractor down here.