Fortunately, each of these 4 units already has it's own forced air furnace and ductwork (I'm not sure about the lines to each unit).Does anybody know - how much would it cost to split one gas meter into 4 separate gas meters?

If the latter, you have to get ductwork into place and that adds to the cost. 

Any estimates on anything like this that anyone has done before as it will need a complete system AND ductwork?

They allow you to finance certain capital expenses like roofs, HVAC, duct work, insulation, solar, etc.

You will also want your ductwork cleaned, along with the airhandler, and filters replaced.If cleaning doesn't solve it, and you still have some residual odors- Deodorize using product specifically designed to eliminate (not mask) odors.

Paid about 4K to fix it up which included installing central heat, along with duct work, since it originally had a floor heater.

Think ,insulating, attaching furring to the interior for drywall, limited design parameterers, running ductwork, and mechanicals, asthetics, etc.

The down side of that approach is that it will impact the tenants a great deal due to the new duct work. 

Sometimes the dust builds up in the duct work and of course on top of the ceiling fans where you can't see it.

Kevin,If the furnace (implying forced air) is questionable, why not leverage the existing ductwork and install a heat pump?