Required for the biosynthesis of the tetrasaccharide linkage region of proteoglycans, especially for small proteoglycans in skin fibroblasts. 3-O-(beta-D-xylosyl)-L-seryl-[protein] + UDP-alpha-D-galactose = 3-O-(beta-D-galactosyl-(1->4)-beta-D-xylosyl)-L-seryl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Cis cisternae of Golgi stack. Belongs to the glycosyltransferase 7 family. b4GalT7