It's all specualtion, but it is my firm belief that the Star Wars episode was the hold up on season 4.
Then.....why aren't we getting Season 5, either?
Or why aren't they selling us neatly camouflaged sets of "Best Moments!" best-of's, thinking that if they gave us five or six episodes, we wouldn't notice the ones that were missing? (LIke Warner did with the Scooby Doo boxes, when the Sonny & Cher and Addams Family episodes were pulled by Viacom?)
It's only in the last year or two that Disney thought Kermit could sell at ALL, after the mild sales of Season 3. Even the movie two years ago was to gauge whether they should just get out of the whole thing in the first place, since it wasn't even their idea (they never could figure out why Michael Eisner was so obsessed with getting them, and he's gone). It didn't help that half of Jason Segel's gags in the movie were condescendngly snickering about what a "70's relic" it was.
It's nice for SW fans, looking for new discussion topics to conspiracize about Disney, to think "They f33r us!", but that's more than 10's Muppet fans can say, sadly.