It'll vary vastly depending the tenants, how many people live in each unit, whether or not they're employed, and whether your fixtures are water efficient.
I have two duplexes with one unit occupied in each at the moment and the bills are vastly different...
I have one unit occupied by a working brother/sister/baby. I imagine the sister's boyfriend is there once and a while also. They use about 0.38 MCF/mo. If you had four of them, it would put you at $242/mo for water and sewer
In my other duplex, I inherited a terrible tenant with a unemployed daughter and deadbeat boyfriend. They're home all day so I see water usage at all hours of the day. That unit uses about 0.8 MCF/mo. If you had four of them, it would put you at $466.65/mo for water/sewer
You should also take into account lawn maintenance. The city will fine you if your lawn grows out of hand. My property manager recommended 4x mowing in the summer (not sure if that's excessive) which comes out to $120/mo