Hiddink wasn't in charge of Chelsea in the cup game. He was in the stands sitting next to Abramovic.
Not that it means anything as I am sure Hiddink will implement the 4-4-2 as well. Chelsea's weakness is their flanks and with both Drogba and Anelka on the filed they will work a lot from the center as both have a high work rate drawing back, especially Anelka.
When Essien comes back I expect him to field a diamond midfield like this:
Essien
Lampard Deco
Ballack
I don't know who can replace Essien against us in such a formation though.