Hydrolyzes the N-acetamido groups of N-acetyl-D-glucosamine (GlcNAc) residues in chitin. Shows activity towards the chitinous oligomers GlcNAc(3), GlcNAc(4), GlcNAc(5) and GlcNAc(6), but not GlcNAc or GlcNAc(2). Requires the substrate to occupy subsites 0, +1, and +2 for optimum catalysis. [(1->4)-N-acetyl-beta-D-glucosaminyl](n) + n H2O = n acetate + chitosan kcat is 7.62 min(-1) for GlcNAc(3) (PubMed:30755482). kcat is 5.59 min(-1) for GlcNAc(4) (PubMed:30755482). kcat is 7.52 min(-1) for GlcNAc(5) (PubMed:30755482). kcat is 9.07 min(-1) for GlcNAc(6) (PubMed:30755482). kcat is 0.097 min(-1) for ethylene glycol chitin (PubMed:30755482). kcat is 0.012 min(-1) for colloidal chitin (PubMed:30755482). Strongly expressed in the midgut. Has little or no expression in other tissues tested. Expressed during the 5th larval instar (feeding) stage with a strong peak of expression in newly-molted larvae, and progressively lower peaks of expression at day 3 and day 7 of the 5th instar. No expression detected during the pupal stage. Strongly down-regulated in response to 20-hydroxyecdysone (20E). Up-regulated in response to juvenile hormone analog. Belongs to the carbohydrate esterase 4 (CE4) family.