The formation of a cross-link between peptide chains mediated by a chondroitin 4-sulfate glycosaminoglycan that originates from a typical O-glycosidic link to serine of one chain; the other chain is esterified, via the alpha-carbon of its C-terminal Asp, to C-6 of an internal N-acetylgalactosamine of the glycosaminoglycan chain. [ https://proteininformationresource.org/cgi-bin/resid?id=AA0219 http://www.ncbi.nlm.nih.gov/pubmed/1898736 ]