Re: Second pumping never yields as much
4 oz is a normal pump session. you're getting lucky on the first time! most mamas produce less for the pump than they do for the baby, and most working mamas don't get enough in pumps during the day to completely compensate ... they add pumps in the morning, weekend, or extras during work. if you're at work 9 hours, in my experience you need to be pumping at least 3 times. 6 hours you might be able to get away with twice.
overall, milk removal is what stimulates milk production, so waiting longer to "build up" the supply before pumping will actually backfire in the long run.
sorry, i know this isn't usually good news, and pumping sucks (pun intended) ... but the solution is pretty straightforward.
you'll also want to make sure your pump is relatively new, in good working condition, and that the membranes etc have been replaced recently.
Nursing, pumping, cloth-diapering, babywearing, working professor mama with the awesomest SAHD ever.