You must have a restriction in your internal pipe work going by your diagram, you are going to need to find this first before considering adding a pumped tank as you could end up with the same problem.
What showers do you have ? If they are non waterfall showers they will only need around 1bar and 10-12l/min to run well so I doubt you will need 50l/min + to run four. I can run three simultaneously with just over 30l/min and 4 bar from the mains (cylinder with pressure reducing valve to 3 bar).
A megaflow cylinder will have a pressure reducing valve on it so you are unlikely to get the pressure required to run four showers simultaneously even if you manage to increase your waterflow