With an SDIRA, you can use tax-advantaged funds to purchase property, and any income or gains from the investment grow tax-deferred (or tax-free with a Roth SDIRA).

This defers capital gains and depreciation recapture taxes on the sale of your fourplex.After converting it into your primary residence, you may qualify for the primary residence capital gains exclusion ($250K single/$500K married), but only for appreciation after the conversion; gains from the rental period remain taxable.

We are taught to defer to the City zoning departments and to direct buyers to that department for their own due diligence. 

Interest income is interest income, unless you decided to roll some of the current cash into a self directed IRA at which point your interest income would be deferred for a later time and allow your funds to grow tax free.

Meaning you use the life-left of all the components that will need replacing in due time and factor that into your offer.Components expiring within a short period of time needs to be accounted for as deferred maintenance.

You state there's a lot of deferred maintenance, then state it is a remarkably well maintained property.

And you would get to indefinitely defer all tax on the gain from the sale.

You do have to purchase at least as much real estate as your net sale if you want to defer all tax.TICS are generally cash deals. 

And the rest of the gain will be deferred in the 1031 exchange.

One of the best advantages about REI is tax shielding, thus you can defer taxes to double net annual income if you do so through passive income.