If your back fits a hasselblad, then it it takes 3.25 x 4.25" pack film, not 4x5. Since Polaroid is no longer, look for Fuji instant film. (B&H sells, among others). If you are limiting yourself to black and white, your choices are either 100 asa or 3000 asa.

Perhaps others can chime in - I can't recall if Fuji plans to discontinue one of the b/w instant films in this size. (I might be confusing it with the 4x5). I've used both with great results.