There is much less 4x5 instant film available now than there used to be. The two peel apart films that I am aware of are Fuji FP-100C colour film, and FP-3000B black and white, but I think that those are both only available in 3.25x 4.25 and wouldn't work in my old Polaroid film holder.

There is a group trying to bring back a version of the Polaroid type 55 film, you can follow them here: http://new55project.blogspot.com/ but I don't think that they have anything ready for prime time yet.