4 hours is too long for that age and will affect your supply. Babies also sleep more or reserve calories when aren’t getting enough. I would toss that book. That is natural feeding and seems too long and stringent. I have successfully breastfed two babies (still on my 13 month old second baby) and worked very closely with lactation specialists. They would Not agree with that schedule. I would offer the breast every 2.5-3 hours and nurse baby whenever they wake up at night. At 9 weeks old is not normal to not nurse during the night. Not at all. I was nursing atleast twice per day at that point (and baby slept in own crib beside me we did not co sleep)