AT&T Inc. flipped a switch and turned on its 4G wireless network Wednesday. The switch, however, was in the company's marketing department.

By relabeling its existing 3G network, the country's second-largest wireless carrier joined the noisy fray over so-called fourth-generation wireless technology, which promises mobile Internet speeds so fast that huge files can be downloaded in minutes and streaming video can be watched without the interruptions of earlier-generation technologies.

As...