UDP-N-acetyl-alpha-D-glucosamine + N-Acetyl-beta-D-glucosaminyl-1,6-(N-acetyl-beta-D-glucosaminyl-1,2)-alpha-D-mannosyl-R <=> UDP + N-Acetyl-beta-D-glucosaminyl-1,6-(N-acetyl-beta-D-glucosaminyl-1,2)-(N-acetyl-beta-D-glucosaminyl-1,4)-alpha-D-mannosyl-R