Without the recipe, and/or knowing what style of brew is being made, it's virtually impossible to say what actions could/should be taken. What was the target OG? Also, I don't think you're reading things correctly. Just because the hydrometer lists 7.9% potential, doesn't mean that's what you'll get. You need to know where the brew will finish, which takes several factors into account.
That being said, I think just adding 4# of brown sugar is a seriously bad idea. You have a few options when it comes to correcting the gravity slip on a brew.
1. Leave it alone and see how it turns out.
2. Make up some solution of DME and water (keep the water part to a minimum), chill it and then add it to the batch. This can cause other complications since you'll need to account for the extra water that you're adding in the batch size/volume. It will also make it more difficult to figure out the corrected OG.
3. Let it age and drink it when it's balanced, or has the characteristics you originally wanted. This is almost the same as #1...
There are more options, but those are the easiest.
Was this an extract batch, partial mash, or all grain?? If extract, chances are that unless you're doing full batch boils, you picked up more top-off water than wort, giving you a false low gravity reading. The temperature of the wort when you take the reading, for hydrometers, can also effect the reading you get. As can large amounts of other particles in the sample you used to get the OG reading.