Hydrolyzes lambda-carrageenan with inversion of anomeric configuration. Does not hydrolyze iota- and kappa-carrageenans, agarose or porphyran. Endohydrolysis of (1->4)-beta-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. Monomer.