3-O-{alpha-L-Rha-(1->3)-[beta-D-Gal-(1->2)]-beta-D-GlcA}-quillaic acid alpha-L-Rha-(1->2)-beta-D-Fuc ester + UDP-alpha-D-xylose = 3-O-{alpha-L-Rha-(1->3)-[beta-D-Gal-(1->2)]-beta-D-GlcA}-quillaic acid beta-D-Xyl-(1->4)-alpha-L-Rha-(1->2)-beta-D-Fuc ester + UDP + H(+) — reaction