One couple, two incomes.
Live on one, save the other.
Buy first 4plex FHA.
Live in one unit, accelerating savings.
Accumulate downpayment for building #2.
Buy building #2 with 25% down.
Resist temptation to increase spending; saving accelerates due to income from building #2.
Buy building #3.
Assuming we’re talking about 4plexes that cost in the range of $800-900k, repeating the above strategy four times results in you retiring with $3+MM (maybe much more, depending on how good the deals were) in equity in today’s dollars 30 years from now.