Because your 4-plex was used partly for your main home and partly as rental, you can only exclude the gain for the portion that is considered your primary residence as long as you considered the home your "primary
residence" for 2 of the last 5 years. This portion of the gain will qualify for an exclusion of up to $250,000 or $500,000, if you file a joint return
with your spouse.
See this IRS link
for more information on the exclusion:
https://www.irs.gov/taxtopics/tc701.html
You will need to determine the portion of the gain from the sale that is for the rental units and report this gain as the sale of a capital asset. In
general, the adjusted tax basis of a rental property is the cost of the
property (i.e., what you paid for the property when you first purchased it),
plus amounts paid for capital improvements, less any depreciation and casualty
losses claimed for tax purposes. Improvements add value to the home, prolong
its life, or adapt it to a new use. Regular repairs and maintenance are not
included in the tax basis of the home.
As for deferring the gain from the sale of rental property, you would only have what is called a "like-kind" exchange under IRC Section 1031 when you purchase a like kind property within a certain period of time. This would only apply to your situation if you used the proceeds from the sale of your rental portion of your 4-plex and purchased another similar type of rental real property.
One of the requirements is that both properties must be held for use in a trade or business or for investment. Property used primarily for personal use, like a primary residence or a second home or vacation home, does not qualify for like-kind exchange treatment.
Please refer to this link for more information about like-kind exchanges:
https://www.irs.gov/uac/Like-Kind-Exchanges-Under-IRC-Code-Section-1031
