No it is Heavy 4. There is no option for twin linking an SMS and the SMS needs no line of sight so you just fire four shots and roll to hit. They are not separate weapons.Neither the Hammerhead or Sky Ray say their Smart Missile System is Twin Linked. Does that therefore make them separate weapons able to shoot individually?
What complicates things is the Burst cannons on the Hammerhead fire as separate weapons. GW do like to make things complicated dont they.Just to elaborate a little bit more; don't go by the models, go by the rules. Each vehicle may be equipped with a single SMS, which follows its profile; it may look like there are more than one SMS on the vehicles, but they are only able to be equipped with the one. Be careful of that!