I am not a guru, but what I have cured has not picked up the dark color until heat is applied. Either smoking, boiling, baking or Disco even wraps his in plastic wrap and steams I believe. The internal meat doesn't darken as much unless injected.
I have already been chastised once today over terminology....LOL But a pickle, a brine, and a cure are like a Mustang, Firebird, and Camero, they are all domestic sports cars but different. That is why I would not worry too much.
As thick as those butts are, I would think 4 days to be a short brine. But...... I are not a guru.
Just my thoughts.