Methyltransferase catalyzing 4-O-methylation of glucuronic acid side chains on xylan. glucuronoxylan D-glucuronate + n S-adenosyl-L-methionine = glucuronoxylan 4-O-methyl-D-glucuronate + n H(+) + n S-adenosyl-L-homocysteine Expressed in hypocotyls, roots, rosette leaves, stems and siliques. Up-regulated during secondary cell wall deposition. No visible phenotype; due to the redundancy with GXM1 and GXM2. Reduced levels of methylated glucuronic acids. Gxm2 and gxm3 double mutants show reduced stem mechanical strength. Belongs to the methyltransferase superfamily.