"Z71" refers to a package of optional extras available for vehicles manufactured by General Motors, while "4x4" is used as a shorthand way to describe a four-wheeled vehicle in which all of the wheels are powered by the engine, rather than just the front or back wheels. The term 4x4 should not be confused with Individual-wheel drive, or IWD, in which separate motors operate each wheel independently.
Z71 and 4x4 commonly appear together in marketing for vehicles sold by both the GMC and the Chevrolet motor companies. However, it is possible to buy a vehicle with the Z71 package without having four-wheel drive included, and the reverse is also true. General Motors offers several varieties of both four-wheel- and two-wheel-drive vehicles, which feature the Z71 package, including the Chevy Silverado, Suburban and Tahoe as well as the GMC Sierra.
Both GMC and Chevrolet have been offering a Z71 package on vehicles since 1988. The 2015 model year vehicles include a fully boxed frame, independent front-end suspension and a wider rear track. These features are intended to improve the towing and hauling capacity of heavy-duty vehicles, as well as to ensure a more comfortable ride and increase vehicle durability. Vehicles featuring four-wheel drive benefit from these features, but they aren't necessary, particularly on low-slung sports cars, which focus on high power-to-weight ratios.