So be careful there.Reasonably modern purpose-built fourplexes like you see in much of the country are very rare.  

Most people practice modern portfolio theory, it's what I was taught when I studied Finance in college.

I'd recommend small local banks for something like this.Your insurance number seems high - I'd shop around on that and combine it with your personal home / auto for a multi policy discount.Regarding your question on security deposits, yes they generally transfer as a credit toward closing.  

If I added a co-signer to the loan, would we be able to combine our DTIs?

We are looking for around 600k.We have combined income of 18k month, 160k to put down credit scores of 700/770 with no foreclosures or defaults. 

rental units.Founder and owner of Another Way LLC, a Real Estate Investing consulting company.RealtorsLori and Doug Davis, Centry 21 Over 41 years combined experience selling Real Estate.Sold hundreds of rental units throughout Wisconsin.Actively working with many other residential Real Estate Investors.Work as a team so much more flexible with scheduling.AccountantRachel Lindemann, MBA Owner of HQ Accounting & Tax Serices, LLC Accountant since 2008Currently works with multiple Real Estate InvestorsExperienced with creating and maintaining LLC’sReal Estate AttorneyJonathan W.

I would expect those combined to be closer to $600 monthly, because you will likely have a lower class of tenants and you will be dealing with 4 separate units, which means 4 separate bathrooms, kitchens, etc to maintain.I cannot get much more specific though, given that I do not know anything about the layout of the building or the specifics of the neighborhood.

And we all know that the first rule for preserving one's mental health and faith in humanity in our modern world is "don't read the comments section!"

Your suggestion along with @Tim Booz suggest of an introduction from a mutual friend sounds like a winning combination! 

If you use a combination of lecture and real evaluations from your market you become succesful and credible to others.