Agree you don't need four membranes but the controls are in place.
Never understood why techs struggle with RO. A production system could be as simple as a membrane, pump, a float switch, pressure switch with a time delay relay, a couple of valves and a regulator.Many manufacturers make one with proprietary waste valves, etc. If you recapture and reuse the reject you don't have to squeeze every bit of efficiency out of it. It's soft and should be fine for rinse or chemical blending. I use my reject to blend chemicals on my istobal.
I've had a booster pump die before and the two XLE membranes would push out over 1GPM on regulated 50 psi street pressure. However It wastes a lot of water if you don't have a way to recapture.