Accumulating cash flowing properties w/o running out of cash

[Matthew Mucker]

How do RE investors accumulate a number of cash flow positive properties without running out of cash?

Let's say I have enough money to put a down payment on a fourplex and I find one that'll cashflow $400/mo. That's 50 months before it generates $20K for me to use as a down payment on my next property. (And realistically I'd probably need closer to $40K down.)

I don't see how investors can accumulate a large number of properties without finding massive amounts of cash elsewhere.

What am I missing in my vision here?