I think you have units confusion.
mm/h is not ml/h
mm is used in irrigation and meteo raining and is the amount of water rains in a square meter. so:
1 mm = 1 liter/m2
1 mm/h = 1l/m2/h
in your case because you have to divide your 4lt/h by the m2 of irrigation area in order to get the lt/m2/h
hope it helps