If you want to leave your corporate job in ~ 10 years, & expect to live off of rental income then you will either have to get used to managing tenants, or you will have to hire a management company. You will also have to get comfortable with taking risks, like your 401k loan. You are building a small business, not throwing money into index funds, and to be successful you will have to move out of your comfort zone & do things that are not fun. That's the honest truth. You will get dirty, you will loose sleep, you will be uncomfortable...success is not easy, but it is certainly doable.
To live off your RE portfolio, you will need cash flow, & it sounds like you are off to a good start with this 4plex. Hire a management company so you can shift your focus & time to growing the portfolio, & away from tenant handling.
Keep the 4plex (don't slaughter your cow, milk it), and pull equity out via a traditional refinance, or (preferably) a HELOC. Find a bank who will appraise the building based on Income, & not comp sales. Reinvest all the profits back into your business, & don't use profits for personal expenses until the day you plan to start living off the profits.
If it is workable for you, consider making your next multifamily purchase as a primary residence, as the financing terms are better, and you can live in it while you are renovating. Then repeat the process, & continue until you reach your income goal.
Here is a good BP article that will help you visualize your end goal:
https://www.biggerpockets.com/renewsblog/go-small-...