My 4month old baby, i'm concerned his drinking too much. a dr once told me he should be drinking 3 hourly. but he doesn't always finish his full feed of 180ml and would continuously drink on that bottle until it's finish?

Hard to assess: An infant will set their own pattern of feeding both in duration and amount. If the kid is filling out, growing well and wetting diapers every 4-6 hours it is probably fine. A 4 mo may go 4 hours between feeds, longer at night &may take more or less at a given feed. If it ever empties several bottles in a row you add to the next.