does this film fit the Polaroid 4x5 back? Do I need the Fuji back?
The short answer is that the Acros film I bought is for ordinary 4x5 film holders; you load these in the dark and then you can use them in daylight in any 4x5 camera with standard back. If you have a camera with Polaroid film holder (back) on it, you must get a standard back to use standard film.
No regular sheet film fits the Polaroid 4x5 back--just Polaroid Instant 4x5, which I think is actually not exactly 4x5.
As I remember (from the past), the instant 4x5 by Polaroid took a back that also took the individually enclosed 4x5 traditional films in a sheath--called film packets. I used the individuals in a Kodak or Fuji holder, but was told they also would fit in a Polaroid holder. But the reverse--using a Polaroid 4x5 film in the Kodak or Fuji holders--would not work because these did not have necessary steel rollers for Polaroid film. So if you can find 4x5 Film Packets (Kodak no longer makes them--Fuji might; I am not sure) you can use those in the Polaroid holder back. Packets are convenient (no loading of film holders in the dark), but they are much more expensive than loose sheet film.
So the Polaroid Instant film and the Kodak or Fuji individual film packets were individually enclosed in packets that were larger than 4x5.
The Acros I bought from you is just regular 4x5 sheets made to load into standard sheet film holders. Graflex made 4x5 film holders that were slightly thicker (but barely fit), and held 10 sheets; after each shot, you pulled a device that brought another film into place.
I doubt you will find a traditional 4x5 back sold by itself because virtually all cameras come with it. The one called a Graflok back could be used normally, but also could be easily removed and replaced with optional backs such as those for rollfilm. If you have a Graflok back with a Polaroid filmholder on it, you can switch out the Polaroid back for a 4x5 holder back. The usual second-hand dealers and auctions may have what you need for shooting 4x5 sheets.