In theory, I'd always prefer a back four against a team playing 4-3-3 or 4-2-3-1.
However, it's slightly different against Chelsea since at least one of their wingers will tend to drift inside at any given time.
Also, I'd be concerned playing a back four with Pirlo in front of it being left to pick up Mata or Hazard. Juve probably need the third (spare) CB to be ready to step forward to confront whoever is in the trequartista position for Chelsea.
I would also say that it probably wouldn't be a great idea to suddenly change to a back four for this game, since Juve haven't been playing that way so far this season.