It's easy to obtain, costs very little, and doesn't require extra, on-going effort to maintain.

Section 8 can definitely cause extra headaches like the longer wait times to get a tenant in there, the inspections, and the incompetent CMHA office you can never get a hold of.  

Fix it up and rehab or flip and then you can pay yourself back and have a little bit extra to push to the next one

If you want to go the extra mile, adding something like Netflix or Disney+ is a nice bonus, especially for families or higher-end guests, but it’s not a must.Cable TV?

I.e. a property sold down the street with 1 extra bathroom and 250sf more.

And more photos taken, same photos, and extra photos of any damage.

Promissory notes in general, as they have no collateral, seem a bit higher risk, perhaps mortgage note investing which are backed by residential and sometimes even commercial real estate would be a safer alternative, and yet very lucrative, too,  8-12% yieldsFor extra returns, I put a small portion of my portfolio into BDCs, Business development companies, that specialize in loaning money to growing businesses which are too high risk to get loans from proper banks.

Without complete financials up to that point, extra work (and headache) will be required to get everything straight after years of failure to keep complete financials.There also might be an asset protection wrinkle by not maintaining complete books for an LLC, but I won't comment on that as I'm not an attorney.

That’s about $900/month in extra carrying costs, but once sold it could potentially be worth it. 

For the 4 plex you are spending an extra 80k in renovations (I question your 110k renovation number to get to 4 units - this could be low but I don't know the square footage or layout).