GO:0050509:   N-acetylglucosaminyl-proteoglycan 4-beta-glucuronosyltransferase activity
Catalysis of the reaction: N-acetyl-alpha-D-glucosaminyl-(1,4)-beta-D-glucuronosyl-proteoglycan + UDP-alpha-D-glucuronate = beta-D-glucuronosyl-(1,4)-N-acetyl-alpha-D-glucosaminyl-(1,4)-beta-D-glucuronosyl-proteoglycan + UDP.