Guerrilla Tactics deals 2 damage to any target.
When a spell or ability an opponent controls causes you to discard Guerrilla Tactics, Guerrilla Tactics deals 4 damage to any target.
Discarding as a cost to cast a spell will not trigger the ability. Only discarding as an effect will trigger the ability.
If it is discarded, the damage done by it is done by this card due to an ability and not by a spell.
The second ability will trigger even on an optional discard caused by an opponent.