I wouldn't be surprised if they had him playing a frog again. Technically he did play a frog in Shrek the Third.
Interesting how there have been at least 2 animated movies to feature a song by Paul and a Frog Chorus.
On that note, if Paul does play a frog, and if he contributes a new song to the movie, I really hope he doesn't do another "We All Stand Together." Blecch. Sorry, I just never liked that song. If you listen to Memory Almost Full, it seems like any one of those songs would fit in nicely in a Shrek movie.