Won Gold on all Playlist 4 Challenges in Co-op Mode.
Playlist 4 has a total of 3 challenges:

(1) Mexico Drift Battle

Gold Medal Requirement: 50,000 PTS (solo) - 50,000 PTS (co-op)
Preferable Car/s: Skimmer
Preferable Mods: Double Acceleration - Drift Boost - Any

(2) Mexico Air Support

Gold Medal Requirement: 20,000 PTS (solo) - 20,000 PTS (co-op)
Preferable Mods: Double Health - Homing Missiles - Any

(3) Hong Kong Survival

Gold Medal Requirement: 06:00:00 min (solo) - 06:00:00 min (co-op)
Preferable Car/s: XNN 233 OR Dodge Charger SRT8
Preferable Mods: Double Health - Double Armor - (Explosive Ammo (as a gunman) OR Top Speed (as a driver))