D.T.
I am the mother of five, pregnant with my sixth, and its been my experience that if they are putting out plenty of wet and soiled diapers, then they are just fine. No one else can ever tell you what is the right size, alot of critics wanna offer unsolicited advice about your baby being too big or too small, but if he has plenty of output (meaning diapers) then there is probably plenty of input. But 4oz every 3hours sounds fine to me.