My question is do I have to run background and credit check on all four adults or just the person paying rent?

You can have additional adults (and children) on a lease as occupants rather than tenants.

They are friends to each other and they are all adults.

An idea I'm having is maybe forming a partnership with my brother, who is a legal adult, and he could sign purchase & sale agreements under my LLC's behalf as well as contract assignments, but I simply am not educated in that area enough to know what exactly I can do.

I am at the pre-screenining process.Every potential tenant has to answer a set of questions before I even show my property as I do not want to waste my time on tire kickers.Potential tenant:has a toddlerhas one college age teen(over 18) who visits herhas another adult child who visits herinherited some money currently lives in the area with her roommate who is also her landlordMy property is:well taken care of A class property priced just below the market in a great area but I do have a high security deposit requirementI am not in any particular hurry to rent to just anyone because I do not need a headache.I do have a pretty good idea that she would not quality based on her income.I do have very strict guest rules in a contract.If they are not guests then they have to be on the application and qualify individually.It already looks like it is a bit too complex.Your thoughts on this?

The family in the inhabited unit was on a month-to-month lease until a couple of months ago, at which point I had my attorney send them a 30-day notice of nonrenewal of lease.The tenants are effectively ignoring the nonrenewal notice, and my attorney has reached out directly to these tenants a number of times in order to offer them more time to vacate, as they are a little older, aren't in the best physical health, and have an adult son for whom they care.My attorney is communicating with the tenants on my behalf, exclusively, as I wanted to be as legally cautious as possible.The tenants are not responding to my concession to allow more time to vacate (they asked for an additional six months - unreasonable - I countered with two months), and they still claim to my attorney that we have a valid lease, despite there being a valid nonrenewal notice having been issued.

They hire high school cleaners with a couple adults at $14-$22 per hour - so they're not doing everything. 

Housing ain't free, this tenant's inability to act like an adult and pay for the roof over his head isn't your problem.

Other option is 3 strikes (all with $ fines) and they are out.They are adults (or close to it) and need to figure out that decisions have consequences.

I have one house where it went from 2 to 5 adults during covid.