I wouldn't personally. Did
YOU 
take a hydrometer reading before
YOU pitched
YOUR yeast? (friend, yeah right)
If you hit your numbers then I would rather have 4 gallons of good beer than 5 of watery....or if you didn't take a hydro reading, then you will simply have 4 gallons of stronger beer.
If you add water at this stage, you are opening yourself to several possible problems, such as contamination, and/or oxygenation.