There are three general categories of backs. There is the 545 series which uses sheet film, it is all discontinued. There are 4X5 back from Polaroid and Fuji that take pack film and there are backs for 4X5 cameras that take the smaller sized pack film commonly used with medium format cameras and Polaroid pack film cameras. The FP100C is the smaller format 3.25" X 4.25" pack in color, there is a black and white version as well. The 4X5 packs are FP100C45 as indicated above.
The smaller is a LOT cheaper and great for proofing. In fact, I enjoy using it for its own sake, and really appreciate that it costs about 1/3 the amount of the 4X5 version ($8.50 vs $26.59 at B&H) In fact, if you have the 4X5 back, it would probably be worth looking for a 405 back on eBay for the film savings alone, if you plan to do a fair amount of this and the size is not as important to you.
In other words, as others have been asking, which back to you have?