Probably better to follow above advice...stick with the smaller 3.25 X 4.25 Fuji pack,
using a 405 Polaroid, or PA-145 Fuji, instant pack back for your 4 X 5 camera.
The 550 Polaroid, and PA-45 backs, for the 4 X 5 instant pack film, still cost over
$125-$150 USD, on the used market. The freshest existing 4 X 5 Fuji pack film will
probably expire in less then 12 months, maybe sooner.
The 4 X 5 costs $3 a shot. The 3.25 X 4.25 costs at, or under, $1 a shot, and
the smaller backs can be had for $40-$70 USD, on the used market.
The only bummer is that Fuji has stopped making the FP100b B&W film in 3x4. they still make the FP3000b, but I really don't care for it. Once my limited stash of 100 is gone, there is no more. Their color stuff is nice, though..
If you don't want to stand behind our troops, please feel free to stand in front of them.