Caveat: I have not done this, I have only done one deal, but this is basically my plan over the next few years.
First off, this is a pretty aggressive timeline.
Second, I assuming you want passive income and are not interested in a wholesaling or flipping business.
Third, I assume you are willing to put in a ton of work in the next four years to get to a passive position, because I don't know another way to do it.
I would BRRRR.
I don't see a better way to gain enough equity to own that many properties.
In my area a good duplex can cash flow $600/month with 75% LTV financing. There is a local hard money lender that offers 70% Loan to ARV financing.
So if you buy a rundown duplex for $100,000, put $40,000 in rehab and refinance it with a $200,000 ARV and a $10,000 down payment you have $600/month. (72% cash on cash)
$600x12months=$7,200/year
$100,000/$7,200=14 duplexes
14/4years=3.5 BRRRRs/year
Maybe you could do 2 4-plexes/year, or just one huge apartment building that takes 4 years. I don't know.
14 $200,000 duplexes at 75% LTV is $700,000 in equity that you got for $140,000. I don't know a better way to get $700,000 for down payments.
Looking forward to hearing other strategies.