What was found
N/A DURING TAXI OUR AND JUST BEFORE TAKEOFF, WE GOT A BUSS POWER FAIL LIGHT AND NR 4 GEN UNPARRALLED LIGHT ON. SMOKE BEGAN TO ACCUMULATE IN THE COCKPIT. TAXIED BACK TO RAMP. FOUND NR 4 GCP TO HAVE A HARD FAILURE. SUSPECT NR 4 GCP AS CAUSE OF SMOKE. RAN A/C PNEUMATIC SYSTEM, CHECKS NORMAL. REMOVED AND REPLACED NR 4 GCP IAW MM 24-20-0 6-2. PERFORMED OPERATIONAL CHECK IAW MM 24-20-00-0, OPS CHECKS GOOD.