None of you guys are entirely correct 
The first number is the total number of wheels . . . or SETS of wheel in the case of dual wheels
A dually pickup truck . . . for example, a crewcab F350 dually . . . would still be 4x4
A 3 axle class 8 tractor where both of the tandem rear axles are driven would be 6x4 . . . 6 SETS of wheels, but only 4 SETS of wheels are driven