Pumping?

Elle • Snap: MochiBunn
Hi- my son is 4 days old today and my milk came in last night. I've been nursing him
On demand but I'm in pain so I pumped one side. 
I pumped 4 ounces from one side- is this normal? Is there more in there? How often do I need to pump to avoid becoming engourged? It really hurts to just do nothing- plus they leak. 
Any tips would be appreciate. I have zero clue what I'm doing.