In Ask Ausellio:
It confirms that PM will be back on April 27, "when Larry comes back from his space-shuttle mission 'a changed man,' teases exec producer Cheryl Heuton, who adds that MacNicol has yet to sign on for the next season. 'We hope he'll return in some capacity. But if he doesn't, we've got some great plans for filling out the college world with interesting guest actors.'"
And when someone copy/pasted that to the Numb3rs forum on TwoP, Cheryl's reply was:
Re: Peter MacNicol --
[spoiler]I fully expect to see him back on the show during season 4.[/spoiler]
We would have liked to lock Peter up for as long as possible. Peter, however, had other plans. Some actors are like that. He's been given to short-term deals for years now, from what I know of his career. Leaves him more choices. Like going to do 24.
And, in addition to TV acting, Peter writes and directs. Shorter term contracts allow him the freedom to pursue those sorts of opportunities.
While I can understand why MacNicol would do this, it doesn't make me particularly happy to contemplate Larry leaving the show any time soon. And forget about the fans: Megan needs him! Charlie needs him! Even Amita needs him, imho.
Weirdly, though, I still trust the creators of this show: if they have to write Larry out at some point, I think it will be every bit as good as the send-off they gave him when he departed for low Earth orbit.
So there you have it. Maybe I'm needlessly over-reacting?