Hi niceandclean
I would imagine that the reason is due to your very high (420) tds. This means that on a smaller system the nonadjustable waste ratio would allow your membranes to foul up very quickly. The advantage of a 4040 system as they have recommended is that you can adjust the waste ratio so that your membrane receives a greater flow of waste water past it. This, along with regular flushing (5-10 mins per tank fill), will ensure that you get a decent life out of the large membrane.