Catalyzes the synthesis of lactosylceramide (LacCer) via the transfer of galactose from UDP-galactose to glucosylceramide (GlcCer) (PubMed:21057870, 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). Plays a role in the glycosylation of BMPR1A and regulation of its protein stability (PubMed:29415997). Essential for extraembryonic development during early embryogenesis (PubMed:20574042, PubMed:21057870). 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 Protein modification; protein glycosylation. Sphingolipid metabolism. Trans cisternae of Golgi stack. Highest levels in heart, brain, liver and kidney with lower levels in spleen, lung and testis. In the brain, expression increases after birth. Up-regulated in subcutaneous adipose tissue during obesity and diabetes (at protein level). Single knockout mice show early embryonic lethality (PubMed:20574042). Mice with conditional knockout in embryonic fibroblasts are born normally, grow to adulthood without apparent abnormalities but have reduced glucosylceramide beta-1,4-galactosyltransferase activity (PubMed:30114188). 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 brain (PubMed:30114188). Knockdown in subcutaneous adipose tissue alleviates insulin resistance and adipose tissue inflammation, increases adipogenesis in high-fat diet (HFD)-fed mice and ob/ob mice and reduces the glycosylation of BMPR1A (PubMed:29415997). Belongs to the glycosyltransferase 7 family. b4GalT5