WASHINGTON — Korean automakers Hyundai and Kia on Tuesday agreed to retrofit more than 4 million U.S. vehicles to address theft concerns and install prevention equipment on all new vehicles to resolve an investigation by a bipartisan coalition of 35 attorneys general. The state attorneys general said the automakers will offer free zinc-reinforced ignition cylinder protectors to owners eligible vehicles, including vehicles previously only eligible for the companies’ software updates. They also agreed to equip all future U.S. vehicles with engine immobilizer anti-theft technology. Hyundai and Kia will also pay up to $9 million in restitution costs to consumers and to states to defray the costs of the investigation. Minnesota Attorney General Keith Ellison said the cost of installing ignition cylinder protectors on all eligible vehicles could exceed $500 million, citing estimates from the automakers.