Investors & landlords

IRS pub 527 page 2 says:

under certain circumstances, you may defer paying tax by choosing to postpone reporting the gain. To do this, you must generally buy replacement property within 2 years after the close of the first tax year in which any part of your gain is realized.

Publication 547 (2019), Casualties, Disasters, and Thefts ... includes

How To Postpone a Gain

• Required statement.
• Replacement property acquired before return filed.
• Replacement property acquired after return filed.
• Substituting replacement property.
• Amended return.
• Three-year limit.
• Changing your mind.

How To Report Gains and Losses

• Business and income-producing property. - this will tell you if you should use the 4797 or 4684.