What's the best way to flush 4 yr old gas from an 86 injected Bronco ?

 

Drain the tank....should be a drain plug. Remove the lines and filters, blow air backwards thru them....and then some cleaner thru the same.

The problem may be that there will always be residue in the tank unless it is dropped and cleaned properly. Sometimes it is cheaper to replace the tank.

 

I don't know of a drain on the tank, I suggest you remove the tank.
Replacement certainly is easier than cleaning it.

Remove the fuel rails, take the injectors to be cleaned, most places will charge about $15 each to clean.
Take some carb cleaner and fill the fuel line from the engine bay, then take some compressed air and blow it out from the engine compartment.
Use the same method on the return line and the fuel rails.