The short answer
Four people rarely run four showers at once; plan for two showers and a sink or the dishwasher: 5.5 to 6 GPM. At a 57°F inlet (a 63°F rise) a 199,000 BTU condensing unit holds about 6.0 GPM and a 180,000 BTU non-condensing unit about 4.4 GPM. With 37°F water (an 83°F rise) the condensing unit drops to 4.6 GPM.
Units in the catalog
Rheem RTGH-95DVLN-3 199,900 BTU Natural Gas Condensing Tankless Water Heater, Rinnai CX199iN 199,000 BTU Condensing Tankless Water Heater and Rinnai RX199iN Condensing Tankless Water Heater.
Products mentioned
Condensing gas tankless · Rheem
Rheem RTGH-95DVLN-3 199,900 BTU Natural Gas Condensing Tankless Water Heater
199,900 BTU/h · 9.5 GPM max
Condensing gas tankless · Rinnai
Rinnai CX199iN 199,000 BTU Condensing Tankless Water Heater
199,000 BTU/h · 11.1 GPM max · Propane
Condensing gas tankless · Rinnai
Rinnai RX199iN Condensing Tankless Water Heater
199,000 BTU/h · 11.0 GPM max · Propane
Questions readers ask
What size gas tankless for 4 people?
Size for what runs at once: about 6 GPM for two showers and a dishwasher. At your winter rise that is usually a 180,000 to 199,000 BTU unit.