With that in mind, I’m considering purchasing four rental units over the next four years to build a stream of income that could help supplement my retirement by age 68.The challenge I’m facing is that the deals I’ve come across so far don’t make financial sense.

It's an estimated cash on cash return given current rental rates subtract expenses assuming 7% interest rate, 10% management fee, 5% repairs, 5% capex and other expenses like mortgage, insurance, tax. it's a estimate to tell you what properties to analyze vs ignoreyou can see the are pockets of negative returns as well as pockets of positive return. this is to supplement the data @Devin Conley provided

Negotiations focused on securing the property at a favorable price, enabling us to achieve a profitable margin in a highly competitive market without requiring additional renovations.

I'd be heavily focused on mitigating that future tax liabilty rather than directly supplementing that income.

I'm passionate about real estate and how utilizing this asset class can enable me to build generational wealth and retire early.

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.

Perhaps a better vetting of the buyers on the BP side will enable better quality leads?

This strategy can be a game-changer for building long-term wealth, as it enables you to leverage your gains without an immediate tax hit.Example: Sell a rental property for $300,000, reinvest in a new property of equal or greater value, and defer capital gains taxes indefinitely.5.

Unless there is a history of STRs in the property already and you have experience with STR, you really wouldn't be able to supplement your income with that. 

I also helped my mother get 2 rentals to help supplement her retirement, mistake #1b.