Trouble is I'm not on a meter so I'm hardly the person to offer advice - so I'm not.
If I were you I would think along the following lines;
If your current r/o is set up correctly (using the right restrictor) then you should use 6000 litres to give you 1500 litres of pure (3 to 1 waste ratio.) Then I would work out the cost of that amount of water using the tarrif that you are on.
Then balance that with the purchase cost of a 4040 (£750.00 including a DI with HF5 membrane - more if you need a booster pump). With a waste to pure ratio of 1 to 1 they say you will still use 3000 litres (although some still advise a higher waste to pure ratio on a 4040).
How long is it going to take you to recover the 4040 cost with water saving? (3000 litres per week).
If you were short of water and you were needing to process water quicker then maybe a 4040 r/o might make sense, but according to my friend who has a 4040 and 60psi water pressure, he can fill his 1000 litre IBC tank in 6 - 8 hours. Under these conditions your 4040 wouldn't do much work a week and would take a long time to recoup the initial expenditure.
We are still using a RoMan 450 GPD r/o ( 3 x 150 GPD membranes) between the 3 of us, but its a problem in the winter.
Also consider that a 4040 will cost more to maintain. The cost of replacing a membrane is much higher than your current r/o. I would also anticipate a high resin useage and cost as well.
Theoretically your 20" prefilters will last twice as long as you 10" ones, but they are more than twice the price.
Also consider that's its a rather big unit, so you need to take extra precautions in the winter to prevent freezing, especially if it is a stand alone unit in a shed/garage.
Spruce