Season 4 certainly isn't perfect, but I think it has some great episodes. I think it's just as good as Season 3. "Krusty Towers," "Krabs vs. Plankton," "Selling Out," "The Lost Mattress," "Whale of a Birthday," and "Ghost Host" were all awesome episodes.
Remember Season 3 had some stinkers as well ("SpongeBob Meets the Strangler," "Ugh," and "I Had an Accident" just to name a few).