How to rate priming sugar per gal of wort

I just made NB Belgian stout and it calls for 5oz of corn sugar for 5 gals.I only have 4 gals because of a spell. How much sugar should I use for 4gal?

Damn witches! stealing your beer with magic!

Simple answer: 5oz/5gal :: 4oz/4gal

Complicated answer: look to a priming sugar calculator and determine the amount of sugar appropriate to both style and volume. (ALL NB kits call for 5oz.)