My credit score and history is great so it’s only my income that’s holding me back, but I should be able mitigate that with a qualified co-signer if I can’t find better work. 

Nevertheless, as with any venture, risks can be mitigated with knowledge.

If so, then the risk is mitigated if you decide to buy this fourplex.If your goal is to retire in 10 years, where are you now?

You cannot do much to mitigate it if this is all you do.

It's pointless to ask "what-if-I-fall" because you will 100%, I guarantee you WILL make mistakes, there will be failings, and as long as you mitigate them into a safe setting so they are survivable ones, like learning to ride a bike in a lawn vs out on a highway, or in REI using an experienced REI Realtor/Consultant and experienced PM's vs on own for everything, your going to get through it and be all the better for the experience. 

That mitigate much of the resale market risk.  

It's always good to have reserves for any rental you have, but I think going this route with a reputable company helps to mitigate the unexpected items.

I was able to mitigate but you really have to understand people and the various reactions to stuff they may have. 

Replaced roof, updated electrical wiring, radon mitigation system What was the outcome?

There are so many variables in the US right now that could affect the outcome of this but in the end, if you build and end up with the amount of equity you think you will have, you should be able to sell it if needed which mitigates the risk IMHO.The bigger the risk, the bigger the reward. :)