I'm trying to find an insurance company that can at least provide a commercial landlord insurance policy for the fourplex, and as a bonus I would like it if they also had a commercial umbrella policy that would supplement the commercial landlord policy.

Fannie Mae is also offering a supplemental loan at a slightly higher interest rate of the original mortgage.  

People often use them either for or to supplement retirement. - people often use these loans for holding cash used to invest.

I am very happy with this experience and will likely continue to look for a few flip deals a year to supplement my rental income.  

Perhaps pursuing a degree in accounting to supplement your MBA could help you in the long run, especially if you focus on taxation and managerial accounting.

With Fundrise that would be their Supplemental Income Core Plan, but you should shop around. 50K is usually the minimum buy-in for what you're looking for, and you have that.

I'm looking to supplement my salary around my schedule.

I plan to work until retirement (hopefully less than 4o hours/wk), so RE is to supplement my income and my other retirement.

Are you trying to find a place that will cash flow you nicely each month or a place that supplements your payments and is nice to live. 

I'm trying to balance building wealth for the future with having a supplemental income.