First off, I don't mean the storylines are childish, I mean the jokes are, there are practically no "adult" jokes in these season 4 episodes, while season 1 still had a good amount of them, and as for Patrick and SpongeBob being more dumb/naive, is not a "stupid" excuse, in fact, it's a very good complaint, it means there personalities have changed! They aren't the same SpongeBob and Patrick!
Now to talk about the story lines, it seems like the humor in the episodes are much different than the older seasons, it's more fast paced, the stories seem to end abruptly, like Dunces and Dragons. They seem to be running out of ideas as well; both "Selling Out" and "Krusty Towers" are similar. Another key difference is the animation isn’t as good as Season 2 and 3, but it is a heck of a lot better than season 1, I’ll give it that.
The reason I'm complaining about the songs, is that they are doing it in almost every episode! Back in season 1-3, they only did it every once in a while, so it was "special." Now they are overdoing it, and the songs aren't really funny or even entertaining comparing them to the older songs.
Sorry, but I don't find those to be stupid excuses at all, I find them to be good reasons why Season 4 isn't as good as the older seasons. Once again, this is just my opinion. We all have different opinions.