If a property is sold while there is still debt-financing in place, then the ratio of income attributable to the remaining financing is applied. 

It seems like a lot of the houses have the same type of design and probably share a lot of the same attributes?

It sets up the basis and depreciation schedules for the new properties using the basis (or proration of basis) from the old property.So if he wants he can then spread out the sales of the 4 new properties and each time he sells one he pays tax only on the gain attributed to that property. 

Thats always a driver of growth/rents as well.I love the 4/2 ranch attributes of the house.

The percentage of income attributed to debt-financing is considered unrelated debt-financed income (UDFI).

Preservation of Tax AttributesThe tax attributes of the deferred gain are preserved throughout the deferral period and characterize the gain included in income on a trigger event.

The only other thing I’ll add is a Fannie Mae appraisal for a 2-4 unit does account for rents as one of their attributes -https://www.fanniemae.com/content/guide_form/1025.pdf.  

If you have already experienced success in investing in real estate, what do you attribute it to?  

I attribute this to the time of year and competitive asking price.

If debt-financed real estate is acquired via an IRA, any income attributable to such investment will generally be subject to unrelated debt finance income tax.5.