Catalysis of the endohydrolysis of beta-1,4-linkages in the backbone of lambda-carrageenan, resulting in the tetrasaccharide alpha-D-Galp2,6S2-(1->3)-beta-D-Galp2S-(1->4)-alpha-D-Galp2,6S2-(1->3)-D-Galp2S.
endo-beta-1,4-carrageenose 2,6,2\'-trisulfate-hydrolase activity