Weirdly enough my little boy is also 4 weeks and on 4oz and until today we've been using number 1 teats but have switched to 0 because he was getting really frantic and gassy after feeding. On the 0 teat he only drank 2oz and fel straight to sleep, which he never does! I wonder if that's because they're working harder for the milk and get more tired and that's what's happening with your little one?
My daughter is 7 weeks and just moved her up to size 2 teats because she was falling asleep on feeds and only taking 2 ounces at a time. She would wake up crying because she is so hungry.
My little one is 4 weeks, and he is still on size 0 or 1 teats on his bottles. When you’re feeding, is the teat completely full of milk? The midwife told us to keep the teat half full so baby has to work for the milk rather than it drip in their mouth and they get lazy, which might make them fall asleep. I also undress my little one if he falls asleep, and I find he wakes up for more milk as he gets a bit chilly