At the end of the "Sequel" song, Walter suggests calling the movie "Muppets Again." "Muppets... Again!" was indeed the working title all through filming, but was changed at the last minute to Muppets Most Wanted.
In the song 'We're Doing a Sequel', Gonzo mentions that they're waiting for Tom Hanks to make Toy Story 4, most likely as a joke. Interestingly enough, Toy Story 4 (2017) was announced later that year (2014) for a 2017 release date.
Muppets most wanted is the 12th Muppet film to be released. Here is a list of all 12 films.
1. The Muppet Movie (1979)
2. The Great Muppet Caper (1981)
3. The Muppets Take Manhattan (1984)
4. Muppet Vision 3D (only viewable at Disney Hollywood Studios (Formerly Disney MGM Studios) and Disney California Adventure in California.)
5. The Muppet Christmas Carol (1992)
6. Muppet Treasure Island (1996)
7. Muppets From Outer Space (1999)
8. Kermit's Swamp Years (2002)
9. It's a very merry Muppet Christmas Movie (2002)
10. Muppets Wizard of Oz (2005)
11. The Muppets (2011)
12. Muppets Most Wanted (2014)