@Sung Yu You can 1031 exchange your fourplex into a single-family home, provided it’s initially used as an investment property (rented for 1–2 years) to meet IRS requirements.

I'd say start the process of securing your HELOC if you havn't done so and spend time analyzing deals and determining what type of investor you are looking to be. 

It is how they determine your interest-rate.

Most cost segregation firms will provide a free cost/benefit analysis quote to help you determine if the tax benefits from the study will outweigh the costs.

Determine what type of property you want to buy - What Zip code, what condition homes, price range, etc.Contact 3-4 realtors that helped buyer agent buy the type of homes you are looking to buy.Then go with the 1 realtor that you connected with the most.

Our unemployment is very low and there has been a trend of increasing population and employment.Do your research and determine where it is you want to invest.

Quote from @David Hori: I've been vetting gurus to determine where an investment would return the greatest ROI. 

How much is yet to be determined.

You need to learn how to analyze property and determine if they are a good investment.Here's a guide that describes how to analyze a property.https://www.biggerpockets.com/blog/rental-property-cash-flow...

Sometimes when examining the entire capital stack, its determined the higher finance charges paired with higher leverage is a winning proposition but only experienced real estate investors should be making that call.