After transferring from primary to secondary I realized that my batch is at about 4.5 gallons. I was thinking of adding .5 gallons of ro water to bring it up to the full 5. Are there any reasons that I shouldn't do it?
Its generally not a good idea to mess with volumes after the beer has fermented out. I would just take it for what it is. Did you check starting and finishing gravity? How far off are they from the recipe you used?
Its also generally not a good idea to post a question like this in the Announcements and feedback forum. It would be better suited in the beginners forum.