.), and also need to consider the cost of vacancies or bad tenants.I also recommend you charge market rate, for yourself and the tenant.

Longer tenancies mean lower turnover and vacancy costs, so two single-family homes usually generate more reliable income and higher net cash flow than a fourplex.Vacancy risk: A fourplex has four times the vacancy risk of a single-family home.

I'm currently using a website that I created 4 years ago, posting vacancies by creating a page and cross linking it to a form in my website for applications.  

Hence, the decision was to make the flight using a single-engine plane.The same logic applies to multi-family properties: a fourplex has four times the vacancy risk of a single-family.

Vacancy is the biggest killer with AirBnb (or any) rentals.  

With an extra vacancy or a maintenance issue or an extra turnover, this is definitely a negative cash flow producing investment.So when you say the rent growth and value growth have exceeded your expectations, I would love to have you respond as to how that is possibly the case. 

A new owner with more capital can easily come in and decrease vacancy rates and increase supply to realize a 2-3x revenue.

See the chart from Fair Isaac Company (FICO) below: FICO Score Pct of Population Default Probability 800 or more 13.00% 1.00% 750-799 27.00% 1.00% 700-749 18.00% 4.40% 650-699 15.00% 8.90% 600-649 12.00% 15.80% 550-599 8.00% 22.50% 500-549 5.00% 28.40% Less than 499 2.00% 41.00% Source: Fair Isaac CompanyAccording to this chart, investors should use corresponding vacancy+tenant-nonperformance factors of approximately 5% for Class A rentals, 10% for Class B and 20% for Class C.To address Class C payment challenges, many industry "experts" are now selling programs to newbie investors about how Section 8 tenants are the cure.

these tenant friendly laws seldom matter because the vacancy is so low the tenants want to stay in their units and realize an eviction will make it challenging to ever find a quality rental in this market.  

My target ROI from cash flow is 10% or higher (after factoring in maintenance, capex, vacancy) and I usually aim for a rent to price ratio of 1% or better.