Catalyzes the synthesis of lactosylceramide (LacCer) via the transfer of galactose from UDP-galactose to glucosylceramide (GlcCer) (PubMed:23882130, PubMed:30114188). LacCer is the starting point in the biosynthesis of all gangliosides (membrane-bound glycosphingolipids) which play pivotal roles in the CNS including neuronal maturation and axonal and myelin formation (PubMed:30114188). a beta-D-glucosyl-(1<->1')-N-acylsphing-4-enine + UDP-alpha-D-galactose = a beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer(d18:1(4E)) + H(+) + UDP Inhibited by EDTA. Protein modification; protein glycosylation. Sphingolipid metabolism. Trans cisternae of Golgi stack. Brain and kidney. Single knockout mice are born normally and grow to adulthood without apparent abnormalities (PubMed:23882130). Decreased glucosylceramide beta-1,4-galactosyltransferase activity seen in the brain of female mice whereas minimal or no reduction in the enzyme activity seen in the male brain (PubMed:23882130). Double knockout mice of B4GALT5 and B4GALT6 genes develop normally during embryogenesis and perinatal stage (PubMed:30114188). However, they show growth retardation and motor deficits with hindlimb dysfunction at 2 weeks of age, and they all die by 4 weeks of age (PubMed:30114188). Axonal and myelin formation are remarkably impaired in the spinal cords and increased immature neurons in the cerebral cortices seen (PubMed:30114188). Glucosylceramide beta-1,4-galactosyltransferase activity and major brain gangliosides are completely absent in the brain (PubMed:30114188). Belongs to the glycosyltransferase 7 family. b4GalT6