What I should also have said is that prefilters for small r/o's usually have 1/4" inlet and outlets, for slightly larger r/o's they opt for 3/8" but for 4021 and 4040 you need prefilters with 1/2" inlet and outlets.
If I take my r/o as an example, I consume a little over 4 litres per minute producing water. If I added a booster pump that would increase to maybe 6 or 7 litres a minute. I remember Alex Gardiner once saying that a 4040 could consume as much as 9 litres per minute with a booster pump. There is no way those smaller bore pipes could keep up with that water demand.
