Inactive serine protease which plays an essential role in the innate immune response against bacteria, fungi and protozoa infection by activating the melanization cascade (PubMed:31765430, PubMed:33045027). In the melanization cascade, acts downstream of TEP1, SPCLIP1 and CLIPA8 to promote CLIPC9 proteolytic cleavage (PubMed:31765430, PubMed:33045027). In the susceptible strain G3, appears to be dispensable for parasite P.berghei ookinete elimination which occurs by lysis (PubMed:31765430). Required for the melanization of Gram-positive and Gram-negative bacteria (PubMed:31765430, PubMed:33045027). Required for the melanization of fungus B.bassiana (PubMed:31765430). May form a heterodimer of a light chain and a heavy chain; disulfide-linked. Secreted into the hemolymph. Secreted as a full-length protein (PubMed:31765430, PubMed:33045027). Proteolytically cleaved into two chains which probably remain covalently linked (PubMed:31765430). Cleavage is induced by fungus B.bassiana and Gram-positive or Gram-negative bacteria infection (PubMed:31765430, PubMed:33045027). RNAi-mediated knockdown causes a loss in phenoloxidase (PO) activity, a reduction in microbe melanization and CLIPA14 proteolytic cleavage in response to S.aureus infection (PubMed:31765430). Also, causes a reduction in microbe melanization and CLIPC9 proteolytic cleavage in response to E.coli infection (PubMed:33045027). RNAi-mediated knockdown in the susceptible strain G3 infected with P.berghei, does not affect the number of oocysts and ookinete melanization; however, severely reduces ookinete melanization caused by CTL4 RNAi-mediated knockdown (PubMed:31765430). Increases susceptibility to fungus B.bassiana-mediated infection but not to P.berghei, E.coli or S.aureus-mediated infection (PubMed:31765430). Belongs to the peptidase S1 family. CLIP subfamily. Although it belongs to peptidase S1 family, lacks the conserved Ser residue within the catalytic triad (Asp-His-Ser) which is replaced by a Gly residue, probably resulting in a loss of proteolytic activity.