I presume you are looking for a new membrane to replace your existing one.
If I presume right, then there are 2 membranes, HF4 and HF5. The HF4 is used for systems with high water pressure, usually booster pump applications and the HF5 is for lower pressure systems.
We have an HF5 membrane in our 4040. The system is not boosted and our water pressure is at 40 - 50 psi. It brings the tds down from 125ppm to 2ppm and produces 2 litres of pure to 2 litres of waste each minute.
I was told by Pure Freedom that at this pressure the HF4 would also work, but I choose to spend a little extra on the HF5 membrane.
Purchase from a recognised WFP supplier. I would be careful buying that sort of thing from an unknown Ebay supplier TBH. I would recommend Daqua.co.uk personally.