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!
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!