There was a thread here earlier on polaroid backs where it was said it was discontinued and I recall reading that somewhere else. Even if it isn't, the cost difference is silly. I just bought a 405 back for my 4x5 to shoot the 3.25x4.25 - close enough for me.
EDIT: Here's the other thread. No source is cited for the information about discontinuance of 4x5, but I'm sure I've read the same thing before:
(there was a url link here which no longer exists)
Instant B & W film FP-3000B 45 SUPER SPEEDY" of film method (peeling) (4 × 5 size, film black and white) Instant Peel Type of apartment will be discontinued soon.
Instant 4 X 5, B&W, peel-apart pack film will be discontinued.
It is becoming increasingly difficult. For this reason, forced out of the 4 × 5 film the same size, will be discontinued as described below black and white film. In addition, the color film will be continue to sell.
That doesn't always mean that a product is dead. I knew of one computer company, back in the day, that set up a line, made so much of a product, tore down the line, built up a new line on that spot, and made a different product, then tore that down and built up another line. Technically nearly their entire product line was out of production at any given time.