Catalysis of the reaction: dermatan sulfate = n 4-deoxy-beta-D-gluc-4-enuronosyl-(1,3)-N-acetyl-D-galactosamine 4-sulfate. This reaction is the eliminative cleavage of dermatan sulfate containing 1,4-beta-D-hexosaminyl and 1,3-beta-D-glucurosonyl or 1,3-alpha-L-iduronosyl linkages to disaccharides containing 4-deoxy-beta-D-gluc-4-enuronosyl groups to yield a 4,5-unsaturated dermatan-sulfate disaccharide (DeltaUA-GalNAC-4S). Chondroitin sulfate B is also known as dermatan sulfate.