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