worried about milk supply

Bellebeauty26

My son is almost 4 weeks old now, and i have recently started pumping. I can usually get about 4oz (i only can pump twice a day), but after that I dont feel full and I'm worried that im not producting enough for my son I'll feed him a few hours after i pump, but he is screaming an hour later hungry again. I'm thinking of giving him a few oz of formula, i just dont know how to transtion him, do i add formula to the breastmilk? or just give him a bottle with just formula?