My guesses for Bonus Features for TMS Season 4 are probably the following.
- The Muppets Go To The Movies
- Rare Muppet Commercials
- New Interviews (with Puppeteers and Writers and other behind the scenes people)
I'm just guessing these suggestions, But they sound like great Bonus Features for Season 4.
Of Muppets & Men: The Making Of The Muppet Show would make a great Bonus Feature for Season 5!