Do NOT add wort to secondary. You'll have decreased the yeast population by transfering it, then adding more wort will only stress the yeast that remain. They'll ferment slow, produce off-flavors, and probably leave a sweet, underattenuated beer.
Also, it would be almost the same work to make 1 gallon of beer to add, I'd just bottle the four gallons and save the work for your next 5 gallon batch.