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 rosette leaves, stems, flowers and siliques. Up-regulated during secondary cell wall deposition. No visible phenotype, due to redundancy with GXM2 and GXM3. Belongs to the methyltransferase superfamily. The predicted gene At1g09600 has been split into 2 genes: At1g09600 and At1g09610.