There are many great articles on this subject (I have no affiliation with any of these):https://www.renofi.com/learn/r...https://www.thisoldhouse.com/h...https://www.investopedia.com/a...That being said, these ROI numbers are arguably more relevant for owner occupants.

Rinse & repeat until you hit the limit of 10 mortgages.It's arguably the simplest way to acquire more properties with the best debt terms available.Since you already have 3 properties, you presumably already have a lot of the necessary experience to execute this strategy (e.g.; market & property analysis experience, due diligence experience, property management experience, etc.).And, since you also have good W2 income, if you want to mix in some other strategies (e.g.; and occasional BRRR, etc.), you can probably do that...though, I suggest that the aforementioned strategy should be your bread and butter (since it's lower risk and less work than ARV-dependent and more complicated strategies like BRRR'ing).As for whether to pay off the debts on your existing properties...this depends on factors like: what are your current debt terms?

Arguably less profits on a single property, but money goes a lot farther there than Denver and we have the opportunity to flip more than one house at a time.

I have done 4 terms deals in the last year (arguably the best sellers market we've seen in a decade) because it helped the seller reach their goals. 

Arguable the most desired private schools in our area, the cost for a child is $23k per year.

They are arguably in better shape than you and me.

@Anthony Barbato I understand the desire to have all expenses covered by purchasing a 4plex, but you're only putting down 3.5% on arguably the hottest asset class in real estate in one of the top 2-3 most desirable real estate markets. 

[Not that I'm against refinancing; it's just that 7% is arguably still high, fixed for 20 years. 

Arguably it increases it.

Wisconsin,  I'm Netting ( before repairs/vacancy/CapEx) ~ $20K/yr with 6 rented doors. with  about $100K spent ( down payments) or arguably less, as i do roll over my profits into new down payments.Outside of simply  calculating/assuming what I'd need to retire, then ensuring that  my ( actual, bottom-line)  NOI covers that.