Otherwise he’d have received numerous requests by now.

. - I have gotten numerous rough bids from subcontractors for plumbing, electrical, HVAC etc.

Second, for the same amount of money that you would put into a single property, you can be diversified across numerous properties, which reduces your single investment risk.

To my knowledge I believe you can only have a maximum of three current mortgages or three investment properties to your name to qualify for the HELOC program.As @Andrew Postell mentioned definitely ask a few local lenders in your current market and they maybe able to give better guidance.If this 4-plex is your personal residence right now obviously the best thing to do is google "introductory HELOC" in your location and there should be numerous results from credit unions and a few banks that offer a very favorable introductory rate and terms.

We've spoken with numerous sellers, but nothing yet. 

But apparently this is common in this neighborhood and each home is identified by a "1/2" as a post script to the numerical address of the home (if that makes sense).

There are numerous reasons investors want to do seller financing and as you've read Brandon's book you've probably learned many of those reasons.One common reason is an investor may not get approved for another loan (depending on how many they have) through a lender. 

In my area granite suppliers are some of the most reliable and easy to deal with contractors.

Sometimes it's ambiguous, but even then, why should I bother with the company that has numerous unresolved lawsuits, versus another company that is virtually the same but has none.

There have been numerous journal papers in QF on the subject of calculating an accurate beta.