It's times like this that it would be nice if Gelfling Waldo of The Muppet Newsflash would pop around again and give us some answers.
He doesn't seem to be so active anymore and his Muppet Newsflash site is very rarely updated these days.
EDIT: Sometimes, I think another possible reason for the delay of Season 4 could be if they're trying to put certain bonus features on it. To this day, I've been predicting that "The Muppets Go to the Movies" might be a possible Season 4 bonus feature--and if that's the case, that would mean more copyrights (music, celebrities, occasional film clip usage, etc.) needing to be cleared.
Regardless, I can still see the main reason for this delay being the same reason why Seasons 2 and 3 took so long to come out--so much legal red tape!
And the fact that we're still in the middle of a major economic recession certainly doesn't help things either.