I have to agree with you on that. Other than Filli Vanilli, I loved Pinkie in Season 4. If anything, she actually gained a few things, mainly not how to let her pride get the better of her, and finding out that she's Applejack's cousin, which was an interesting plot twist.
When Pinkie's flanderized, I'm more amused by it, because I find it funny most of the time. I usually think that she's doing it just to be fun, because Pinkie is NOT stupid in the slightest.