There are two options. First, you can get a 4x5 camera and the Polaroid 405 or Fuji PA-145 back. Both of these take the 3.25"x4.25" film and use the full frame. The backs are hard to find, and when you find one, expect to pay $100-$200 for it.
Your other option is to buy a converted Polaroid 110A. It's not a 4x5, but it is a folder, and will give you full frame on 3.25"x4.25". If you don't need/want to swap lenses, this is an easy way to get good quality and full frame: http://www.polaroidconversions.com/packfilm/
I don't mean to hijack this thread. But can anyone point me in the direction of the fuji press release stating the end of color 4x5 pack film? I knew black and white was done for last year, but only heard about color yesterday. Such a shame, barley even got to use my 550 holder :/.
I don't know much about the back mounts for 4x5 cameras yet, but if I couldn't just buy the back I wanted somewhere then I'd just buy something like the osciliscope camera back that I just bought, or a really cheap packfilm polaroid with poor plastic optics, etc, that I won't feel guilty chopping up, then chop up the back/camera as needed and graft it onto something that can clip to whatever camera I have.
I don't know how well it works as a focusing glass but I recently took an expended packfilm cartridge, took it apart, and taped some tracing paper over the window where the film would be. Then I can just insert it into a camera, leave the back open, and hopefully use it to focus. So even if you just chopped up and duct taped a polaroid holder to a camera, you might be able to open up the back, insert the front frame from an empty pack film cartridge covered with something frosty-like, and not have to take the polaroid back off the thing.