Choose an arena. This turn, each of your units that are in that arena whenever one of your opponent's units in that arena is discarded gets an additional +4 power until end of battle.
The effect applies to the arena, not to the units in the arena. If a unit with Hidden Cost deploys to an arena after this card is played choosing that arena, it will get +4 power if an opponent's unit is discarded from that arena this turn. Each time an opponent's unit is discarded from the chosen arena, all of your units currently in that arena get +4 power. If the Hidden Cost unit was deployed after an opponent's unit was destroyed, it would not get a +4 power, because it wasn't in the arena when that bonus was gained.