If we are playing 4-2-4, it means that we will use 4 CMs, considering the fact that we already know who the 3 are (Pirlo, Marchisio, Pazienza), by bringing Vidal would mean that we are selling Sissoko AND Melo.
But can we actually pull that off, considering our horrible history with sales, it would be wiser to sell at least one of them, before we go for it.