My BB was 3.2 kg at birth, drank 2oz milk (3 hrs apart). So it is not abnormal to drank less than 4 oz in the first month. He is growing good, M size within the same age group!
Don't worry! My situation is the same. I haven't add powder milk in the hospital as I tried breast feeding. So, the wweight loss is high. So, she was arranged to attend medical consultation at MCHC for few times. I quitted breastfeeding at once and feeded her with powder milk. Now, she is at normal size lu!