NEW YORK, Oct. 27 -- AT&T has officially announced its plans to split into four separate companies by 2002. The breakup will create an independent cable company and an independent wireless company, all operating under the AT&T brand name. The other two businesses will be the core of a new AT&T Corp., consisting of the unit that runs the company's huge telecommunications network and serves business customers, and a separately traded subsidiary containing the shrinking consumer long-distance business. The breakup is the third major restructuring for the former national telephone monopoly since 1984's court-ordered breakup, Twelve years later AT&T voluntarily split itself into three separate companies, spinning off its communications equipment arm and acclaimed Bell Labs research unit as Lucent Technologies Inc., and its computer division as NCR Corp.