First step if you haven’t, even if zoning would “allow” 4 lots is to look at minimum lot sizes along with setback requirements from the streets and lot lines, among any other restrictions, to see if 4 houses could “fit” into 4 subdivided lots.

Regular ADU permits would not work as the setbacks would make the rest of the lot unusable.  

What are the set back requirements from property lines?

Another con to be aware of is regulatory risk.

That maybe market risk, tenant risk, property risk, regulatory risk, etc.

So to answer your question, first con is regulatory compliance costs.Second: financing considerations. 

I think you should be good on the setbacks and FAR because the property was built not too long ago and the plans show the same habitable space

Anytime you're adding square footage with Denver you'll need to go through zoning and make sure you meet the setback/bulk plane requirements and are still within the lot coverage requirements. 

This small set back is a set up for something much greater!

Get his license number and look it up at your state regulatory agency.