1) You have tenants paying below-market rents in the 4-plex because it has a lot of deferred maintenance, and you presumably lack the money / inclination to do the maintenance because of rent controls. This is a cycle that might only get worse, as the maintenance needs pile up, issues start compounding, and the place gets slummy. When you eventually fix the 4-plex, or when rent controls are lifted, you will probably need to find some new tenants which will cost you some vacancy. If you DIY things, you could lose more revenue from a 6 month project than it would have cost to have it done by professionals in two months. However, keeping the 4 plex nice enough to generate decent rents will be key in the long run to not being stuck with the worst tenants. Maybe plan your way through this minefield and decide if you need to take our yet another equity loan to rip the band-aid and get it done quickly. Rent hikes could easily justify such a move.
2) Your health and umbrella insurance costs seem low. ACA plan?
3) As others noted, you are earning a higher return than safer, hands-off passive investments can generate, but you're doing so by taking on some very specific risks. In particular, uninsured natural disasters, real estate prices, nightmare tenants, neighborhood risk, and legal risk. I would consider each of these as a specific challenge and work on formal mitigation plans for each. FIRE is not only about cash flow, it's about risk management (a topic MMM tends to wave away as pessimistic whining). If your retirement plan is just waiting for the first misfortune to be thrown off, you're no better off than the people chasing dividend traps. Yet there's also opportunity here; if you can mitigate these risks better than others, you will have earned yourself a quick FIRE.
4) When I add up your equity, it looks like your NW might be in the $1.2-$1.4M range. I would be very tempted to cash everything out and do a standard 4% retirement in a LCOL area at $40k-$50k annual spending. However the path you're on now probably has a higher potential for payout and you mentioned family. It's still something to keep in the back of one's mind, considering the other risks. It would be a particularly tempting option if we entered an environment where housing is expensive and stocks are cheap.
5) I agree that except for selling there's not a good way to reduce your real estate risk and shift it to the stock market (where risk is a lot easier to hedge if you know about options, stops, etc.). However selling might not be a bad idea for the right price. You could put the duplex on the market for a few months at an insane price like $1.5M or $1.6M and treat it like a lotto ticket, at the low cost of having to deal with people's inquiries. If you got out of the duplex with $800k in equity I could point you to some preferred stocks yielding 6%/$48k per year, with no work involved. That might retire you on a cash flow basis WHILE YOU STILL HOLD THE HOUSE + FOURPLEX. However, your dirt-low mortgages are a form of golden handcuffs. I would have a hard time letting them go.
6) As others noted, you need to set aside some serious money for repairs, vacancies, etc.
7) Because the 4 plex is on the same lot as your SFH, and on the same mortgage, it seems like that really complicates things from the perspective of keeping your business and personal finances separate and organized. Am I right to assume you co-mingle everything? If so, that could be tricky when you are planning your taxes, trying to depreciate some items but not others or segregate your home vs. business interest expenses. It also complicates any plans to form a LLC. At your point in RE empire building, you need to have solid financial tracking, unimpeachable taxes, and some kind of corporate liability shield to protect your stocks from tenant lawsuits. Some of the other landlords on this forum may have advice on how to account for the benefits of personally using the SFH.
