Most 4040 membranes require a feed pressure of 100psi or 6.9bar to get the advertised rejection rate, so you would be best off looking at a booster pump as mentioned by Jonny 87. You may have a membrane that is 'low energy' and can operate at 5.5bar, but these tend to be a lot more expensive and again your feed pressure is still way below what is recommended.
In terms of your question about when the RO membrane needs replacing...
1. if the TDS is too high for pure water cleaning (usually replace at 10-20ppm, no more).
2. if the amount of pure water being produced has dropped off (i.e. your tank isn't filling as quickly as you need it to).
Some interesting things to keep an eye on...
Low pure water production + OK TDS = likely scale formation inside membrane, can sometimes be revived by cleaning with citric acid. Feeding an RO with softened water is recommended, usually 100ppm of hardness is the maximum recommended by membrane manufacturers.
High pure water production + poor TDS = likely chlorine burn on the membrane, either through not changing carbon pre-filter often enough or by carbon pre-filter being undersized for the flow rate.