The Arcing Weapon feat allows the Acid Arrow to used through your weapon in one of two ways.
1) Melee attack. You do weapon damage and spell damage as normal.
2) Ranged attack. The spell affects one target once. You add the weapon plus to the damage by the spell.
Applying to Acid Arrow, you either get weapon+spell damage (melee) or plus+spell damage. If acid arrow lasts 3 rounds, the spell would normally do a total of 6d4, so an ranged Arcing would do 6d4+4 not 6d4+12.