Between the military, task apps, new business, etc. there is never a better time to be able to make supplemental income and be my own boss.

Find a sales gig or something to do on the side to supplement it.

Those that really succeed in any business either learn from the ground up or they use other experts to supplement what they do know.

I am looking to work in finance/banking for 2-3 years and save up as much money as I can as I pursue my REI dream.I really wanted to either get my salespersons license or brokers license to supplement my income on a part-time basis and for me to get to know the industry better + build connections. 

For generic numbers sake (not based on anything real), $1600/month cost for bothI net $400 per door off of $1200/month rent on the duplex or $800 per month$200 per door on the fourplex off $600/month rent still $800 per monthThen if (when) the rental market declines that $1200 is likely to drop further than the $600/month leaving me with better cash flow on the fourplex, right?

Those generic forms are not well used state by state.

I'm a looking at making a purchase in the Inglewood, CA area and hoping to supplement with rental income - either getting a roommate in a SFH or acquiring a multi unit property.

Even with a live-in hack, the money you're paying in is (a) Building your equity in the home, and, (b) If you're renovating while living in, you'll get the tax benefits, as well as the force appreciation, more equity for you.Eventually you can rent it out for a stable cash flow and refinance on the original home to utilize for your next property.Or, you can always entertain the idea of a MFH (less than four units) - you live in one, and use the potential income to supplement qualifying for a mortgage.These are very loosely-given suggestions, and your due-diligence is of the utmost importance.

Is this a one off thing for them, or are they looking to supplement their monthly income from the interest payments you're making them.

I mean buy a generic house that most people could live in.