All cost-modifying items are buggy on "or" cards. Yuumi, Answered Prayer, Scaled Snapper, Quicksand etc. In determining whether you can play the card, multiply the modifier by 2. When you actually pay the cost, it only takes the cost you would expect (unless you couldn't afford the higher cost! then you get to play the card and it just takes whatever mana you have - e.g. yuumi with mana potion can be played even if you only have 1 mana left). So at 8 mana it would let you play him for 6.

You also see this 'display' bug with other items, for example if an "or" unit has a pickaxe then both choices will look like they have +4 but will actually be summoned as +2.