@Vincent Plant - several suggestions:1. send it again with receipt (ie, USPS delivered it without tenant signature)2. email a copy 3. post on door and take several pictures: 1 up close to show it's the letter, 1 back a bit to show the door (and hopefully house number).   

My thought process has been that renting has allowed me flexibility, no maintenance costs, the ability to move freely if need be and I am able to rent in a great area/ best spot in town while still saving money.

Take a look at these articles:https://streeteasy.com/blog/my-landlord-hasnt-cashed-my-rent-checks-is-that-cool/https://www.brickunderground.com/rent/nyc-landlord-wait-cash-rent-checkI would send a letter with copies of your unpaid checks to her asking for confirmation of rent paid; you'll want to state in your letter that your attempts to each her by text were not responded to and ask how she would prefer contact be made with her in the future. 

I am not re-writing it though because it is good advice for someone purchasing an apartment complex.Since you are only buying a 4 plex, you should have asked for and received all 4 copies of the current leases during due-diligence.

When the prospective tenant orders a background check and credit report, does Cozy send a copy to you or does the tenant forward you  the report?

A real problem is the copy on your door hangers does not match the audience you are giving it to thus generating no leads. you can hand out a million door hangers and if your copy sucks, if you do not know your audience you will get 0 leads.

Send him a copy of his lease or just sign it.

You also need a copy of the lease/rental agreement. 

So, for example, you could withdraw some or all of your contributions to invest in rental real estate, while leaving the earnings in the account.However, if you are serious about intending to eventually purchase rental real estate, then I suggest you save money into an account that you can freely use when the time comes, such as a properly structured whole life policy, a savings or checking account, a money market account, or CD.

And copy the BRRR at a faster pace.