Hi Austin,
It's so nice to see an investor considering the environment. As a rabid environmentalist, I applaud you. However, all of my discussions with people regarding geothermal retrofits have suggested payback times over a decade, which is simply abysmal.
You'll likely do better with solar power- I'd discuss it with a few of the big names in the industry(Solar City comes to mind, but there are many others as well.) I tried to put solar on several properties- one issue I ran into was that the electric service is seldom in my name, other than for the hallways/common areas, which didn't justify the cost of installation. The solar companies are figuring out ways around this, but the I deemed the hassle(and the small return on my time) to be too high a hurdle.
Some properties can benefit from the installation of air source heat pumps for heating in the winter and cooling in the summer as well- they typically look like large air conditioners. If your property is in Iowa, it is likely too cold for air source heat pumps to be very sensible. A state or two south, however, and you'd be very smart to install these- it would put the heating bills in the hands of the tenants, and would give them control over how hot or cold to make their apartments in the winter.
So now to the recommendations. If I were you, I'd install a high-efficiency gas furnace(they can reach the mid-90% range for efficiency.) If you're replacing an old one, its efficiency is likely in the low-mid 80% range. If you could break it into four zones(one for each apartment,) you might do even better- sometimes when you only have one or two zones(for four apartments,) the hottest apartment may crack a window to regulate heat, and that is money down the drain. The payback time you'll experience(you should calculate the cost as the difference in price for a minimum efficiency boiler(around 80%) and a high eff. boiler) will slaughter the payback time you'd experience with a geothermal retrofit.
But why stop with heating? If you're using incandescent lights(or even the swirly compact fluorescent lights,) you'll save a bunch of money and labor by switching to LED bulbs. This is especially true in common areas which may be lit 24/7. I've linked a 100 watt and a 60 watt replacement bulb below. These bulbs generally last for many years in an always-on state, or a few decades with regular household usage, and I recommend using them in your apartments even if you're not paying the lighting bill so that your tenants save money(and can pay rent more easily.) In an always on state, the payback for these(relative to incandescent bulbs) is usually under 3 months.
100 watt replacement: GE Lighting 13909 Energy-Smart LED 16-watt, 1600-Lumen A21 Bulb with Medium Base, Soft White, 1-Pack
60 watt replacement: Feit Electric - 60 Watt Replacement - Omni Directional - LED Dimmable - 3 Pack
And why stop with lighting? I've had great water savings on the installation of low flush(1.28 gallons per flush or less) toilets. I highly recommend using these, along with faucets with the WaterSense label(here's a link to aerators which accomplish the same thing without high installation costs. These things cost a few bucks apiece but can have pretty meaningful impacts: Simply Conserve Two Pack of Low Flow 1.5 GPM WaterSense Standard Faucet Aerators) You'll save on both water costs and the cost of heating the water as well. Low flow shower heads can be trickier- if your tenants aren't happy with the shower quality, they may become unhappy with their apartment- so be careful with low flow shower heads.
Finally, I'd make sure your insulation is up to snuff. If your attic insulation is degraded due to age, or has too low an R value due to lower standards from prior years, fixing that can have a very rapid payback time.
After you've made many of these energy improvements, don't forget to let future tenants know that they're renting in a building with high-efficiency fixtures- the yuppies go wild for saving the earth and such(translation- cha ching!)
Best of luck!