Goblin Barrage

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

You can't cast Goblin Barrage unless you choose a creature as a target, even if it's kicked. However, you can target a Goblin or artifact creature you control and then sacrifice it to pay the kicker cost. The target player or planeswalker will be dealt 4 damage.