The Quad Turrets is an upgrade for the Nod Hub defense systems. Purchasing this upgrade adds an extra turret to the central hub of the defense system. It costs 1,000 credits and is available from the Operations Center.
"Following the Third Tiberium War, Nod started to re-evaluate its defense systems. Realizing that the already formidable power of their Hub defense system had only one weakness, Nod engineers came up with a plan: protect the fragile central hub with a fourth turret analogue to the one of the main system, effectively increasing the power of the defense system by 33% and making it less vulnerable."