Removes maltotriose and maltotetraose chains that are attached by 1,6-alpha-linkage to the limit dextrin main chain, generating a debranched limit dextrin. Hydrolysis of (1->6)-alpha-D-glucosidic linkages to branches with degrees of polymerization of three or four glucose residues in limit dextrin. Glycan degradation; glycogen degradation. Belongs to the glycosyl hydrolase 13 family.