The difference is the cylinder heads. A 2v comes in GT's, Crown Vic's, Explorers, etc. 2v's have 2 valves per cylinder for a total of 16 valves. 4v engines have 4 valves per cylinder (32 valves) which equates to better breathing and more horsepower. 4v engines came in Cobra's, Mach 1's, Mercury Marauder's, and some other specialty vehicles.