Breast feeding question?

My son is in the NICU and the nurse told me if I feed him for more than an hour he would actually lose weight than gain weight. He already lost some weight but they have him on every 4 hour feeding schedule as well. I looked online and a newborn should be eating every 1 1/2 hour to 3 hours. What should I do, do I feed him when he wakes up or wait til the 4 hour or just wake him up every 2 hours?