S.C.
Personally I NEVER switched to the medium nipples, he was on the slow flow nipples until he gave up the bottle. (At 11 1/2 months he decided he wanted water in his cup or milk from the breast, no more bottles!)
Don't worry about how many ounces, let him decide, he knows how hungry he is! If he acts like he's still hungry after the 4oz, have his caregiver offer another 2....