My properties there have been pretty stable with very little turn over although I would highly suggest doing a thorough tenant selection as with any rental.  

Any design/functional suggestions would be much appreciated!

Again there are a lot of exceptions to my suggestions of what I look for but at least you get one perspective from fellow investor.

I would keep them on a month to month , but suggest a $150 rent increase during your initial call.

I should clarify that I didn't mean to suggest long term gains are't important or worth considering. 

I would suggest looking through the SBA website for more information and always check the credentials of someone claiming to be a licensed real estate agent or broker online in the state website where they claim license.  

My in laws have a house that they are planning to let go to foreclosure, my suggestion was a subject to deal.

The last thing I would note is your suggestion to add a 5th unit.  

Withoutknowing the details I can not suggest which would be right to add on.Several you shouldpay attention to are:- Ordinance & Law: Provides additional building coverage to deal with rebuilding costIncreases due to changes in Zoning or Building laws- Personal Injury Liability: Libel, defimation of character, wrongful imprisionment, etc.

If a rough estimate is all you need, I'd suggest doing a Google search as I'm sure there are plenty of sites that will give you such a number.