The function of TM9SF4 (transmembrane 9 superfamily member 4, Ensembl gene identifier ENSG00000101337) is as follows. Associates with proteins harboring glycine-rich transmembrane domains and ensures their efficient localization to the cell surface (PubMed:25999474). Regulates the assembly and activity of V-ATPase in colon cancer cells via its interaction with V-type proton ATPase subunit H (ATP6V1H) and contributes to V-ATPase-mediated pH alterations in cancer cells which play an important role in drug resistance and invasiveness of colon cancer cells (PubMed:25659576). Plays an important role in an atypical phagocytic activity of metastatic melanoma cells called cannibalism and is involved in the pH regulation of the intracellular vesicles in tumor cells (PubMed:19893578). {ECO:0000269|PubMed:19893578, ECO:0000269|PubMed:25659576, ECO:0000269|PubMed:25999474}.