If your 4-plex is residential rental real estate and you replaced the HVAC in one of them, then it doesn't qualify for SEC179. Since an HVAC would become "a material part of" the property it gets classified as residential rental real estate and depreciated over 27.5 years. Property classified as such does not qualify for SEC179. Additionally, since it cost more than $2,500 it doesn't qualify for safe harbor de-minimus either.
