Quick take
For four bathrooms, roughly 11–15 GPM service flow is a more realistic target than 9 GPM for many homes. SpringWell CF4 is 12 GPM for 4–6 bathrooms; RKIN CS9 is 11 GPM for 2–5 and CS10 is 15 GPM for 4–6.
Why 12+ GPM often fits better
Two showers, a faucet and an appliance can easily create a meaningful simultaneous load. More service flow reduces the chance that filtration becomes the bottleneck.
SpringWell sizing example
SpringWell rates CF4 for 4–6 bathrooms at 12 GPM service flow with 1-inch connections.
RKIN sizing example
RKIN rates CS9 for 2–5 bathrooms at 11 GPM service / 17 GPM peak and CS10 for 4–6 bathrooms at 15 / 23 GPM.
Choose between 11–12 and 15 GPM by real demand
A modest four-bathroom home may be fine near 12 GPM. Large rain showers, body sprays, fast-fill tubs or frequent simultaneous use can justify the 15 GPM tier.