The third season was a year long series about one enemy the Xindi, and various battles against it. In fact, the third year wrapped up everything so fully anything done afterwards simply wasn't and never could be good enough.
Enterprise was very good for the first three years with most of the first two years leading to the Xindi battle and the third year focusing on various parts of that battle. Once the Xindi battle was over, there was no attraction to return each week to see the next development in the story line. The first three years were mostly focuses on the Xindi theme either in leading up to the third year or concluding that particular story 'arc' at the end of the third year.. During the first three years, there were few 'elevator episodes' to hold down costs. Indeed, one such elevator episode with Commander Tucker and Lt. Reed stuck in Shuttlepod One was truly outstanding and highly regarded for it's character acting.
The fourth season lead off with the alien Nazis and never went anywhere. It had lost it's focus and had no distinct goal. It seemed like most of the year was a bunch of unrelated episodes which were unable maintain ones interest after the conclusion of the Xindi adventures.
Good points.
I was thinking about the alternate universe Tholian web episodes which i thought was great, the explaination of the change of foreheads of the Klingons and the green Orion slave girls to be pretty enjoyable.
A lot of people believe the Xindi story arc killed the show, i thought it was great fun.
The Shran episodes were good too.
Shran was a great character.