I'm an asset-based (hard money) lender from Atlanta with an extensive 15-yr background in residential and commercial real estate looking to learn more from the BP community and how I can help those that need investment financing. 

I am hoping to be an asset to experienced investors, with the intention of learning as much as I can, so that I can start building a portfolio of my own.

That 4% mortgage is a real asset.

If you can rent 3 units for $2,000 you are close to breakeven.On the plus side, you will have a highly attractive asset and several decades without much cost of capex and repairs. 

The high end is the 20 or so co ops on 5th Avenue or Park Ave (one is on East End Avenue) that require liquid assets of $100 million + to gain board approval.  

But, it’s not a business because even if successful, you don’t own the underlying asset, so unless you grow very large you have nothing to sell.  

A mortgage lender can help determine if refinancing or tapping into equity might allow you to acquire additional properties.Consider a 1031 Exchange: If you're open to selling a current property, a 1031 exchange can defer capital gains taxes while enabling you to upgrade to a larger or more profitable asset.

A financial advisor or real estate pro could help you map it all out more concretely.If I were in your shoes, I'd probably lean towards selling and reinvesting into some more diverse assets, or just more properties, but that's just me. 

Occasionally if they have other assets maybe she can keep the equity of the house and he gets to keep his full 401k or something like that.   

When you build enough capital/assets/net worth to afford a reasonable monthly cushion you can see if the lifestyle of a full time investor is something you’d want to pursue.