From your previous posts, it sounds like you're pretty handy since you are able to gut rehab your own primary. A 4 plex IMHO is about the same or easier so the technical maintenance stuff is already taken cared of. The major roadblock would just be learning how to landlord which honestly, if you are of someone of average or above average intelligence should not be that hard. And honestly, I've been landlording for 6+ years and mostly a piece of cake with occasional hassles. Also, it sounds like you have the financial strength to weather some possible setbacks. With that in mind, I think you can do it and you should go for it. You can only look back on this 5 years from now and think.
1) Damn I went in over my head and got screwed, I lost $X of money
2) Don't do it and wonder whether you could have made it. Then harbor some regret (or not).
3) Succeed, replace half of your income, feel accomplished, get bragging rights along with many other benefits
So depending on how much money it's possible for you to lose, is it worth the possible regret? If the cost is low, I would say go for it.
I, myself, have been in your shoes and have felt the self doubt. But if you are someone who is willing to learn and grow and do, then I think the chances for success are high. Play the odds.