Can I really retire early on 4 duplexes?

[Jen Lucas]

My husband and I are DINKs with a plan to retire early, well go to very part-time, that seems too easy, and I'm hoping the experts (you!) can help us vet this plan.

I'm 39 and my husband is 46. We live in Austin, TX and our lifestyle is pretty simple.  $2,800/mo covers all of our necessary expenses. We don't want to be rich, we want more free time. 

We currently have the following properties, all with mortgages and at least 25% equity. We manage the properties ourselves (family helps with Dayton). 

1. Our home in Austin- SFH, mortgage $1500/mo

2. A duplex in Austin - value $250,000, cash flows $1100/mo after PITI (doesn't include vacancy or repairs)

3. A duplex in Dayton, OH - value $110,000, cash flows $624/mo after PITI (doesn't include vacancy or repairs)

Our plan is to buy 2 more duplexes in Dayton (Austin has gotten too expensive) with similar numbers to the one we already have, so 2 more cash flowing $624/mo.  We have the cash for the down payments and are actively looking.  

Once we buy 2 more duplexes, the total cash flow (before vacancy and repairs) will be $2,972 - enough to cover our monthly expenses. We already have 6 months PITI for all 4 properties in savings, one year's living expenses in taxable investments (can get to the cash if needed), and small retirement accounts.

We both plan to work part time for challenge and social engagement, and money to cover travel, pay down mortgages by the time my husband is 60, AND to cover repair and vacancy expenses.  Once mortgages are paid off, our total cash flow will be $6,142 (in today's dollars), more than enough to support us and cover vacancies and repairs.  We will also have our taxable investments, retirement accounts and social security.

This seems too easy.  What am I missing?  I'd appreciate you poking holes in the plan, giving advice, and pointing out what I'm missing. 

Thank you in advance!