Why exactly would it make it so much easier?
Because it offers clarity on who each player should be picking up.
If you have three CBs and two wingbacks against one striker and two wingers, you need specific plans as to what you want to do with your spare defender.
Does the spare CB step forward to confront a midfielder? Does one of the wingbacks ignore the winger - leaving him to a CB - and try to hold a position higher up the pitch?
You can also get forced back to the edge of your own box because you are forced into playing a back five, so don't have the presence in midfield to defend higher up the pitch.
With a back four it is clearer: the two CBs deal with the CF and each fullback is resposible for the winger.
In the first game against Shakhtar Juve didn't have a clue what was going on. They just ended up sitting deep and hoping. They were being overrun in the middle because the wingers were drifting into the middle and Juve's wingbacks didn't know what to do. They ended up staying wide and basically playing against no-one, so really weren't contributing.
It does, but the team did look very rusty and even awkward with four at the back against Torino.
I'll take your word for it, because I wasn't really paying attention to the game.
At least with the system you are suggesting Juve could easily change to playing a back six.