Catalysis of the reaction: D-glucuronyl-1,3-beta-D-galactosylproteoglycan + UDP-N-acetylgalactosamine = N-acetyl-D-galactosaminyl-1,4-beta-D-glucuronyl-1,3-beta-D-galactosylproteoglycan + UDP.
uridine diphosphoacetylgalactosamine-chondroitinacetylgalactosaminyltransferase I activity