Details:
Catalysis of the reaction: D-galactosyl-N-acetyl-D-galactosaminyl-(N-acetylneuraminyl)-D-galactosyl-D-glucosylceramide + CMP-N-acetylneuraminate = N-acetylneuraminyl-D-galactosyl-N-acetyl-D-galactosaminyl-(N-acetylneuraminyl)-D-galactosyl-D-glucosylceramide + CMP. [EC:2.4.99.2, MetaCyc:2.4.99.2-RXN]