I have to agree with previous posters. Routine is for a sake of parents but a baby has to be also taken into a consideration. If he is not ready for 4 hour schedule then don't force him into it. It works much better for breastfed babies when they start solids, then they get additional nutrition from that. At that point, feeding every 3 hours should not feel as a lot if you compare it to every 2 hours where he used to be until recently.
Also, are you always able to be without food and drink for 4 hours, do you always get hungry at exactly the same intervals? Probably not, that's why I would suggest waiting with a 4 hour schedule until he is a little bit older, especially if he is so great sleeper at night. My dd was not on 3 hour schedule till she was something like 4 months old and we never really went into a 4 hour thing. She never slept well, so I would say, you have a baby that many moms would dream to have :lol:
BTW, I really have very low opinion on pediatricians as they rarely know anything sensible about breastfeeding. Breastfeeding is not the same thing as formula feeding, there are different dynamics to that. Suggesting to put a breastfed baby on a 4 hour schedule will work if the baby is cooperative but not because of his age or weight.