A few years ago, Infiniti faced a genuine challenge. The luxury sport-utility market was experiencing solid growth, and thus was ripe for an entry from Nissan's upscale division. But economic realities dictated the use of an existing vehicle, the successful Nissan Pathfinder, as a base. This dilemma wasn't confined solely to Infiniti, of course, as Lincoln, Lexus, and others were bound by the same constraints when preparing the Navigators, LX470s and the like. Infiniti's timing couldn't have been better. When introduced in 1996, the Infiniti QX4 was based on an all-new Pathfinder, one of the better buys in a slightly lower price class. A crisp exterior shape, rattle-free unit-body construction and a host of design and engineering virtues could be kept intact, requiring only the addition of features Infiniti buyers would expect. And that's what happened. While the QX4 does exude something of a best-of-Pathfinder air -- and there's nothing at all wrong with that -- significant differences more than qualify the QX4 for head-to-head competition with its rivals in the $35,000-$40,000 class.