Hi @Jonathan Cook, you could look into an FHA loan which allows you to put down less than 20%. Otherwise, you could look into non-conventional loan options (outside of Fannie/Freddie) that allow for less than 20%, such as local credit unions.
As for your numbers, you're missing a few important components; the big items would be maintenance, CapEx, and utilities (water/sewer/trash) for a 4-plex. I usually carry 5% of the gross rental income for each of the first two items, and the utilities piece varies depending on location and the building. You'll need to verify which utilities are owner provided versus tenant provided. For example, most 4-plexs for individually metered electricity and gas therefore the tenant pays for them, however the owner still covers water/sewer/trash which can be a couple hundred dollars per month depending on the area. I would also request Estoppel agreements from the current tenants to verify that the $550/month lease amounts are accurate.
Aside for the numbers, it seems like a good owner occupied 4-plex to get into, but I wouldn't bank on pocketing the difference between the rental income and the $810/month.