it goes like this
1 - sediment filter
2 - carbon filter
3 - booster pump
4 - ro
the booster pump should always be after the prefilters as it puts unnecessary stress on the housings, let the pump draw the water through the prefilters (which works better anyway) and then from pump to ro housing. Another reason for this is that as the prefilters age you will notice a pressure drop across the filters, therefore if you have the pump before the prefilters then you will lose 10/20/30psi by the time the water reaches the membrane - on the other hand if you set it up pump after prefilters then there is no pressure drop before membrane, only feed rate into the pump inlet slows down, this is evident in a reduced product output.