I love the Simpsons and still watch it. I probably learned more from that show than any other source of knowledge in my life.
If they need to end it soon I would say carry on until season 5 and do another movie (make sure it is actually good). I think it is best they plan a good ending ahead of time just in case something bad happens.
http://www.youtube.com/watch?v=F1gWECYYOSo
Please Watch/Share this video so it gets shown in Hollywood.