There's plenty of free sage advice here on BP if you spend some time exploring the forums, bookstore, podcasts, etc.
It struck me that 4-12 units is an odd range. One of these things is not like the other.
2-4 Units = Residential property for financing purposes
5+ Units = Commercial property for financing purposes
You might consider spending some time exploring the difference between residential and commercial financing options, and researching/interviewing lenders and loan product options for both. Then you may end up with two different sets of search criteria, for example:
3-4 Unit Properties in a certain price range/location/property class (which will require a relationship with a residential lender), or
5-12 Units in another price range/location/property class (which will require a relationship with a commercial lender, and possibly even a different real estate agent/broker, depending on your market etc)
Having both options lined up in advance will allow you to pull the trigger on either if/when the right deal comes along, rather than having to start from scratch if one piece is missing.