One definition is when each property can't sustain itself.

Also considering going forward and having many properties in the coming years, is it a sustainable source to get payments through with many properties or should I use something like rentredi or Cozy to limit confusion with my records?

On a 4 unit, there is a self sustainability test for the property (just an FYI).You deserve kudos for being up front with your lender. 

With the federal deficit projected to hit $985 billion in 2019, I'm curious what anyone might think of the long-term sustainability of HUD's appropriations for section 8 vouchers.

The current rental is sustaining itself is like a nest egg that I can't blow the money on.

They are self-sustained with rental income.

It seems like a great sustainer of the rental market.

If the markets can sustain a sense of uncertainty and fear for the entire year, without us actually seeing asset values decline, or mass unemployment, the yield curve could stay inverted for all of the next 12 months, and the spread with mortgage rates could normalize.

So to blanket statement saying never invest in negative cash flow may be something that resonates with you but should not be the rule of law for investing. 

Last week, a WI insurance agent gives me quote that the building cost there is about $250 sq/ft - $300 sq/ft.Honestly I don't know how new homebuilder could sustain this, if they have to buy land and build after 2023.