So I've just finished the whole series! Hooray!
I guess I don't have much to say that hasn't been said before. The third season is one of the best seasons of television I've ever seen. I don't think Star Trek- ANY incarnation of it- has had that strong a season. Maybe DS9 at it's peak.
The first few episodes of S4 were really confused and kind of bad. Then it picked up in the middle and became about as good as S3.
Peacekeeper Wars was pretty good but
PS- I think it's horrible that this show got cancelled. A lot of really interesting territory was started to get mined in S4. The mirror universe type stuff especially.