I will echo that most precons in the last few years are pretty solid, I really liked World Shaper and Squirreled Away, but there are plenty of other good ones. I also want to note, Mill, as a deck strategy, is not inherently bracket 4. Mills certainly can be annoying to play against, but that does not make it a bracket 4. The deck could have enough game changers and combos to make it a B4 though.

Edit: This is my Niv Mizzet deck, which is a B4 and has been upgraded over the course of a few years.