Yes, you can use a Polaroid back on a 4x5 camera, but unless you want to be scrounging for film on eBay, be sure to get the sort of back that accepts 4.25 x 3.375 (image area: 3.75 x 2.875) pack film such as FP-100C or the just-discontinued FP-3000B. The full 4x5 packfilm and sheetfilm that go in the larger 4x5 Polaroid backs are no longer available fresh from anyone.