I love Airbnb, but we are looking to invest in future properties in a less continuously labor intensive way. 3 properties are easy enough for me to clean myself but beyond that, we would need to hire cleaners more regularly and eat into our profit. 

FHA security instruments require a borrower to establish bona fide occupancy in a home as the borrower’s principal residence within 60 days of signing the security instrument, with continued occupancy for at least one year.Source: https://www.hud.gov/sites/documents/4155-1_4_SECB....

Couldn't you just deduct money from the previous tenants security deposit towards this?

When you buy your regular landlord's policy for your 4-plex it will come with dwelling coverage, which give you protection in case of fire or other property damage, as well as "Premises Liability".

Channel, frequency, firmware, software, security settings, noise in the environment, something as simple as moving furniture around can impact coverage. 

I'm assuming you know the basics of paying in full, on time, etc.Here's a few things that will help fast track to establishing credit -Get an auto loan / a secured loan. 

My term is 1st+last + security. 3 times rent of about 900$ is not something ordinary people (not rich but "working") should have?

I believe you can put down as little as 3.5% and even if you just secure a duplex, you could live rent-free (off the rental from the other side) and could save up faster for your next purchase. 

This would also afford me the security of having around $6K cash flow monthly coming in from my business should I have some unforeseen “bumps in the road” with my rookie real estate investing but also afford me true potential to grow my real estate portfolio.Plan B is to sell my business and use the cash to purchase several multi-unit properties or even an apartment.

I don’t want to block the other "regular" rent option and my realtor is trying hard but he is not rental agent so can't deal with section 8.