For the FMV as of the date of the previous owner's death, you should be able to get one through a qualified real estate appraiser. They can give a retroactive appraisal based on records of market conditions and sales of similar property for the year of death. Your maximum tax would be the difference between the selling price and the FMV at date of death, but the gains could be reduced depending on how the ownership was structured, the fact that part of the property is your own residence, and so on. You can also take into account the cost of any improvement that you made since inheriting the property. But also, the cost basis starting with the FMV has to be reduce by depreciation taken during rental of the other 3 units. Hire an expert and split the cost.
