Lightning Surge deals 4 damage to any target.
Threshold — If seven or more cards are in your graveyard, instead Lightning Surge deals 6 damage to that permanent or player and the damage can't be prevented.
Flashback
(You may cast this card from your graveyard for its flashback cost. Then exile it.)
To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.