This was pretty evident when the ratings started dropping during the clone/amnesia/New Krypton arc (which I really liked--there I admit it).
I would hate for others not to want to watch the series because they only think that the first two seasons are the only good ones.Dean's portrayal of Clark/Superman is the best as far as I'm concerned.Even though Superman was a superhero he never felt he was above anyone. He was incredibly sweet and romantic, not to mention yummy on a stick.While Lois is still very much her tenacious self, Clark Kent was now a confident, well-adjusted guy and was a far cry from the bumbling Clark of the films and comics.
True it does start to lose some spark in the middle of the 4th season but I think that was more the network's problem than anything else.
Dean Cain and Teri Hatcher had incredible chemistry throughout the whole series.