Ok thanks spruce ro was working great then we moved house and it's un unusable now it's a hf5 membrane so I thought it would still work with low water pressure 
Our water pressure is between 40 and 50 psi and we operate a 4040 with an HF5 membrane without a booster pump. The issue associated with low pressure is also flow rate. (Pressure and flow are different.) If the pipe is heavily calcified, or has some other restriction, then performance is going to be a problem as these units need a good flow of water.
On flush mode our water flow through the system is about 13 litres per minute through the prefilters when new, but I don't know what it is directly out of the tap though.
Even although the town we live in has had new piping, water pressure and flow are very different throughout the residential areas. We live at the end of a water main ring on our estate, so our pressure is less than our neighbours behind us who are on a larger main ring.