UDP-glucuronosyltransferase (UGT) that catalyzes phase II biotransformation reactions in which lipophilic substrates are conjugated with glucuronic acid to increase the metabolite's water solubility, thereby facilitating excretion into either the urine or bile. Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds. Involved in the glucuronidation of calcidiol, which is the major circulating form of vitamin D3 essential for the regulation of calcium and phosphate homeostasis. Also glucuronidates the biologically active form of vitamin D3, calcitriol, probably leading to its biliary transport and intestinal reabsorption. glucuronate acceptor + UDP-alpha-D-glucuronate = acceptor beta-D-glucuronoside + H(+) + UDP calcidiol + UDP-alpha-D-glucuronate = calcidiol 25-O-(beta-D-glucuronide) + H(+) + UDP calcidiol + UDP-alpha-D-glucuronate = calcidiol 3-O-(beta-D-glucuronide) + H(+) + UDP calcitriol + UDP-alpha-D-glucuronate = calcitriol 25-O-(beta-D-glucuronide) + H(+) + UDP Homodimers. Homooligomer. Interacts with UGT1A1, UGT1A3, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 to form heterodimers. UGT1A4 is one of the isoforms produced at the UGT1A complex locus. The UGT1A complex locus produces different isoforms based on alternative use of promoters, first exons and terminal exons. Belongs to the UDP-glycosyltransferase family.