I don't think there will ever be a Redwall movie. But I don't think a fourth, officially produced, series isn't out of the question entirely. Stranger things have happened. And as far as I know, the show was never entirely or officially cancelled. They just never got around to producing a fourth season. I guess what they need is renewed interest, or just enough investors.
I watched the show when it was on PBS in 2000 when I was a little kid, I re watched the episodes on youtube.
A group on DA is working on Mossflower as a fanmade production as we speak. They have scripts and they got people to be voice actors, the drawings are amazing.
I think It'll come out good.