Yup. It is an absolute crying shame they don't make 4x5 instant film anymore. I'm not sure why either. It was plenty popular (and useful) for "professional" studio photogs as well as "landscapers". I know I used it and loved it. I would use the type (52?, 55?, can't remember) that provided me both a print *and* a negative. You would have to expose differently to optimize the quality of the print or the negative, but you'd still get one of each from each exposure that could be used. Of course, I would optimize for the negative and use it for printing later (I think most of us did that).
The Polaroid 450, 500 and 545 backs are for the discontinued 4x5 instant films. I'm fairly certain there were some others too that were 4x5 only.
I think the Polaroid 405 back will accept the still-produced pack films. The image size will be smaller than 4x5, but its still plenty useful. I don't think there's a pack film that provides both a negative and print.
Fuji also made backs, analogous to the Polaroid versions (for both 4x5 and pack film formats). You can find any/all of these backs today. In fact I think Fuji still produces their pack film back so you could get a brand new one. Otherwise, lots of used backs out there. Obviously, the 4x5 format backs are dirt cheap, and the pack film backs are rather spendy.
I really wish they'd bring back the 4x5 instant films. I have a 500 and a 545 back, both in great condition (oh, you will want to pay close attention to the rollers on a used back. They MUST be perfectly clean and free of defects -- smooth and shiny). Anyway, if the film comes back, I'm prepared to start shooting more large format. Have yet to use the 90 SA on a "real" subject. I think a 90mm on a Crown Graphic or Speed Graphic 4x5 makes a quite nice "field camera" for landscapes.