Quad Heavy Bolter

If a shooting weapon has the twin-linked special rule, or is described in a model's wargear entry as twin-linked, it re-rolls all failed To Hit rolls.

For a Blast weapon, if the scatter dice does not roll a hit, you can choose to re-roll the dice with a Twin-linked Blast weapon. If you choose to do so, you must re-roll both the 2D6 and the scatter dice. 

Twin-linked Template weapons are fired just like a single weapon, but must re-roll failed To Wound rolls and armour penetration rolls.