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.