the 4 attack is the base attack and the -12 wall is a bonus conditional attack which FOLLOWS the base attack. so overall in the best case you can achieve 16 damage. However, it delivers the bonus attack AFTER the base attack, so if your opponent plays basic wall, then he will just lose the 10 wall. the bonus coming after the base attack is because this card is a very cheap common which can deal quadrable damage to its cost.