Breastfeeding & Bottle still not enough??

Alyson • I have a 1 year old son and currently pregnant again due May 5th 2016 ♡
My son is 18 days old. I breastfeed 15-20mins on each breast and my son still acts like he's hungry so I supplement 4oz of formula well after that he's STILL acting as if he's hungry. I don't know what to do. Does any other mommas experience this?