You can, but you probably shouldn't. You'll dilute both the gravity and the flavor. Next time start with more and/or get a bigger vessel. All my five gallon batches start as five and a half. Minus sediment (I've got enough head room to not have blowoff loss) I usually end up with five.
If you do add more water, make sure to boil it first. You don't want to add an infection.
But I wouldn't dilute it.
As suggested by durrettd on your other thread, it wouldn't be a bad idea chill it to the low end of the yeast's range. That's what I do.