New Water Heater and Venting

I am contemplating installing a new gas water heater that will have a 4" vent. The existing water heater has a 3" vent. Both the furnace (4") and the water heater are vented through the same pipe (6"). I can't find the GAMA tables that everyone references to see if I will be within spec.
Can anyone help this newbie? Any advice would be appreciated.
Thanks, JJ
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