I can say the show still has some wackiness to it. There's been the over-dramatized death of a spatula, Karate Island really being a timeshare sale setup, SpongeBob being able to drive only while blindfolded, everyone freaking out that Gary will turn them into zombies, etc. But like Spongey34 said, it just isn't as "natural" and so it isn't that funny. There's also not enough SpongeBob/Patrick goofiness and them interacting with (or should I say, annoying) Squidward. Patrick seems to be written just to deliver jokes now instead of act them. One thing I don't like about Plankton is that he's seemed to have lost his intellegent side I loved about him ("I went to college!", "Felicitations, malefactors! I am endeavoring to misappropriate the formulary for the preparation of affordable comestibles. Who will join me!?").
WHERE HAVE YOU PEOPLE BEEN??? It's a mind game just because Steve has left now. This started WAY before season 4. I Had an Accident, The Camping Episode, New Student Starfish, Missing Identity, SpongeBob Meets the Strangler (all Season 3 episode) have VERY similar humor to Season 4. It didn't just start now.
I see what you're saying and I agree that the show started a pretty small decline at the end of season 3. However, most of those examples I feel aren't right as I feel they still seem like the show. For examples, I would use Born Again Krabs, Krusty Love, The Great Snail Race and Plankton's Army. As for Steve leaving, I've decided not to blame the decline on anyone, really. People are always unsatisfied with the new. Season 2 was an even more drastic change to the show than season 1, and while most of us say it's a change for the better I'm sure there were people who didn't like the new style. The change was likely because of new staff, but you also have to remember they took a two-year break (all of season 3 was finished in 2002; season 4 began production in August or September 2004) so being away from it for a while can really be different coming back to. Then again, I think the movie was just as wacky as the rest of the show, so shouldn't two years of working on that still keep the zaniness in the show? I really don't know.
As for the Squidtastic Voyage plot, it's been done on Fairly OddParents, Jimmy Neutron, Magic School Bus, The Simpsons, Rugrats, and of course the original source, the movie The Fantastic Voyage itself. And as for your petition, forcing the crew to return to the "classic style" seems pretty immature and meaningless because it'll do nothing, so just give up on it. There's been nudity since episode one so what makes you hate it now?