The spectator selects 4 cards while the performer is facing away.
After selection, the performer turns his back toward the spectator.
The spectator shuffles the 4 cards and holds 2 in each hand.
With performer's back turned towards the spectator
He is able to call out the sum of the values of the card in each hand.
He proceeds to identify the cards.
And even calls out which card is in which hand.