K.A.
A baby's stomach is about the size of their fist. At 4 weeks old that's maybe about the size of a marble so it can't hold very much per feeding. A few ounces are really all they can take at a feeding.
A pump isn't as good at getting milk from the breast as a baby. How much you pump is no indicator of how much milk you are in fact making.
http://www.kellymom.com/bf/supply/index.html This page at Kellymom has a lot of wonderful information about pumping moms, milk supply, how to tell if baby is getting enough etc. Kellymom is my favorite internet site for breastfeeding help!
Congratulations on your little one! You can do it!