I have a 5 gal batch of beer that actually came out to just a bit over 4 gallons--always check the stamped measurements on your new brew pots! Mine was 3 quarts low.
Anyway, we are bottling this weekend. I assume that I'd just reduce the 3/4 cup priming sugar in proportion to the volume of beer I am missing.
If it matters, we are using 32 oz ez-cap bottles.