The Goblin Hexer's Vexing Cloud is a standard action with a zone:
Area burst 3 within 10; automatic hit; all enemies within the zone take a -2 penalty to attack rolls. The zone grants concealment to the goblin hexer and its allies. The goblin hexer can sustain the zone as a minor action, moving it up to 5 squares.
If the Hexer centers the blast on himself and it grants him concealment, do melee attackers then take an additional -2 to hit because of the effects of the cloud.
To break this down, assuming that concealment grants the Hexer +2 bonus to defense, and the cloud make its attacker take a -2 to attack.
Does that mean that the attack modifier from the cloud is -4? (-2 concealment + -2 cloud effect)