|My friend has one... I think it's the TZ4, but it may be the TZ3. It's a slick little bugger. The lens is quite sharp (Leica) and I haven't seen any of the NR issues in low ISOs like the reviews indicate. It's pretty solid, too, his has been dropped several times and it just keeps running. The menus aren't very pretty imo, and it lacks some of the really neat, unimportant features that some Canon ones have (I love the Color Swap and Color Accent feature, for whatever reason!), but it's a good little camera. The one significant thing I'd ask for from it is a bit of a better Macro mode.
As for the differences, they're all mostly the same. The megapixels vary I believe, and I think the newest one actually has a slightly larger sensor than the others. I'd check DPReview to confirm any other differences, though.
He got his from Microcenter for about $350, I think, when it was new, so $239 is a pretty decent deal.