It looks like, if you convert from MCF to kWH, for the equivalent energy consumption you'd pay $41 in electricity vs $9 for gas. That seems like a no-brainer.
The next question is how to distribute the heat. You can either use duct-work or hot water.
With Duct Work you'd use a natural gas furnace and obviously run the hot air distribution lines and cold air returns into each unit. The downside to this is duct-work takes a lot more space and needs to be factored in to the design of the four-units. Installing multi-zones on a furnace so each apartment can control their own temperature can be challenging and more expensive than with a boiler and hot water. Double check the cost on this too.
The other thing to consider when installing ductwork, and you'd have to talk to your local building official, is penetrating through fire separations. Where we build in Canada, each unit needs to be separated with a 1 hour fire rating. If you're running ductwork through the fire separations it will cost you more to have the correct fire dampers in place. Just something that I'd suggest asking about before committing to this course of action.
With hot water, you can do either hot water baseboards or in-floor heat. You'd install a natural gas boiler for these systems. The advantage of in-floor heat is it runs at a lower temperature (110-120 F) vs hot water baseboards which run at (180 F) meaning it should consume less energy to operate. There are also lots of aesthetic advantages of in-floor heat, and you may be able to charge a premium rent for in-floor heat. The disadvantage to infloor is you'd need to do a concrete over-pour on the 2nd floor. (You can run the in-floor heat pipes between the joists, but you lose a lot of heat into the floor cavities and have to run at a hotter temperature. I don't recommend this solution.) Both in-floor and baseboard are easily zoned so each unit can control their out temperature.
More than likely the easiest and most cost effective method is to install a natural gas boiler and hot water baseboards throughout the building.
Have you checked with any local installers yet?