What kind of water heater set up for quad?

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Hi, I have a single family home that currently has 1 tank water heater serving 3 bathrooms and a kitchen.  I am converting to a duplex and later on into a quad. The contractor is recommending adding a second for the duplex. My question is, what are my opinions? I need to make changes to the current water heater now anyway, I'd like to get it set up for the quad so we don't redo work later. Do I need one heater for each unit? is there a single commercial water heater I can use for four so I only need one unit? Space is an issue and we would need to run new gas lines for tankless.