Quote from @Scott Mac: It might be the investing environment.Right now we have a different interest rate picture then we had a few years ago.

Unsecured personal debt is often from institutional sources, but at rates of 9-20%, fully amortized over 3, 5, or 7 year terms (not interest only).

Are you getting the property at market rate or below market rate?

Seller finance the property- ask for an up front, non-refundable down payment of $XXX (which could be used as his primary residence down payment), payments of $X over X years at X interest rate with a balloon payment in X years2.

., deductions and depreciation), and an inflation hedge, as rental income and property values often rise with inflation.

A 4 bedroom would have you competing with larger condos or townhouses with pools, a much lower rate of return. 

Keep looking, but be prepared for higher interest rates with maybe maximum of 65% loan amounts (hopefully more though).

In this case, your monthly payment with 20% down will have a good MARKET rate if your DTI works with a much lower month payment and no mortgage insurance(MI).

Do you think this strategy gains strength if lets say... in five years interest rates are at 5.5 vs 7.5 now?  

You can often get better rates with a national lender, but the customer service is non-existent, communication probably sucks, they may ask for the same documents multiple times, etc.