I guess you could ....
I would heat up some water and sugar until clear, then add.
You don't want the sugar to chrystalize in the brine..it should be smooth and clear.
4 Cups of sugar does seem like plenty of sugar for sweetness, I wonder what happened...usually mine are very sweet, even too sweet rather than tart
Did you use Cider or White vinegar ? Maybe that is it.
Good luck, let us know how it goes