Plasma membrane-anchored serine protease that directly induces processing of pro-uPA/PLAU into the active form through proteolytic activity (PubMed:24434139). Seems to be capable of activating ENaC (By similarity). {ECO:0000250|UniProtKB:Q8VCA5, ECO:0000269|PubMed:24434139}. (Microbial infection) In gut epithelial cells, facilitates human coronavirus SARS-CoV-2 infection through, at least, the cleavage of coronavirus spike glycoproteins which activates the glycoprotein for host cell entry. . This is the function of Ensembl gene identifier ENSG00000137648 (TMPRSS4, transmembrane serine protease 4).