As you are mentioning the 4% wort shrinkage, I assume you are using brewing software of some sort.
During the brewing session, you will lose volume for several different reasons:
- Grain Absorption. (The brewing software should handle this)
- Dead space, and wort trapped in equipment. (This should be constant)
- Trub (and hop absorption if you use whole hops). (This will vary according to the grain bill and amount of hops)
- Evaporation (This will vary slightly depending on the weather, and boil vigorosity. I know there is no such word, but you understand it). It will also depend on the boil time, but the software should take that into account.
Carefully measuring each of these items should enable you to check your brewing software setup, which in turn should get you in the ball park.
Then do what GilSwillBasementBrews
recommended to accurately adjust the volume for each brew. (I don't do this because I fly sparge. I sparge until I reach the required volume or until the gravity of my runnings drops below 1.010, then I make up any required volume with water.)
You should try to end up with at least the batch size + 4% at the end of the boil. I also need some extra for trub and hop absorption. Actually, I know I need 7g pre-boil with 2 oz hops, and an extra 12 oz for each extra once of hops, and I end up with between 5.00 and 5.25 gallons in the fermenter.