You are over thinking it.
Mine is a 4 stage and I added a 2nd membrane thats all. Now its a 5 stage.
Again you dont need 4 canisters unless your incomming tds is very high.
Mine is a 4 stage and I added a 2nd membrane thats all. Now its a 5 stage.
Again you dont need 4 canisters unless your incomming tds is very high.