How many pounds of carbon dioxide would you prevent from entering the atmosphere in one year, if you were to replace a 100-watt light bulb with a cfl bulb that uses 12 watts in a fixture that you have on 4 hours per day?

That's an excellent question but we don't have enough information to answer it. We would need to know how the electrical energy is generated. Nuclear, hydroelectric, coal burning, wind, and natural gas burning all produce different amounts of carbon for the same amount of electrical energy. If the utility that supplies your house uses nuclear or wind, then you can leave both bulbs shining 24/7 and they're not responsible for any carbon.