You may be able to capitalize on this if your area has experienced similar growth.Simplification: Selling would allow you to focus on your new single-family investment properties and potentially expand in those markets.Capital for other investments: The proceeds from selling could be used to fund other potentially higher-yielding investments, especially if you use a 1031 exchange.Small town limitations: Depending on the specific town, there may be limited growth potential compared to larger markets or more popular tourist destinations.Given the stable cash flow, low remaining debt, and your established experience with the property, keeping the fourplex seems like a solid option, especially if it's in a location with STR potential.

BP has various tools to help you analyze your deals (rentals, flips, wholesale, etc). : https://www.biggerpockets.com/investment-calculators7.

Does your family member leverage a dynamic pricing tool like PriceLabs?

Is fixing and flipping in Charleston, SC popular?

The BiggerPockets' "Find a lender" tool is a great option for connecting with reputable, investor-friendly lenders. 

It’s awesome that you’re diving into creative financing with the subto strategy—it’s a great tool for investors.

Any other tools you’d recommend?

I'm seeing parts of Durham become increasingly popular like 27703 for new construction in addition to Bethesda and South Durham near Southpoint Mall. 

Second In-N-Out Burger Coming to the Strip: The popular burger chain, In-N-Out, is opening a second location on the Las Vegas Strip at the new BLVD development, across from City Center.

I know Craigslist is very popular for renters and landlords in SF so I assume the same would be true with Berkley.