The function of MTMR4 (myotubularin related protein 4, ENSG00000108389) is as follows. Lipid phosphatase that specifically dephosphorylates the D-3 position of phosphatidylinositol 3-phosphate and phosphatidylinositol 3,5-bisphosphate, generating phosphatidylinositol and phosphatidylinositol 5-phosphate (PubMed:11302699, PubMed:16787938, PubMed:20736309, PubMed:27625994, PubMed:29962048, PubMed:30944173). Decreases the levels of phosphatidylinositol 3-phosphate, a phospholipid found in cell membranes where it acts as key regulator of both cell signaling and intracellular membrane traffic, in a subset of endosomal membranes to negatively regulate both endocytic recycling and trafficking and/or maturation of endosomes toward lysosomes (PubMed:16787938, PubMed:20736309, PubMed:29962048). Through phosphatidylinositol 3-phosphate turnover in phagosome membranes regulates phagocytosis and phagosome maturation (PubMed:31543504). By decreasing phosphatidylinositol 3-monophosphate (PI3P) levels in immune cells it can also regulate the innate immune response (PubMed:30944173). Beside its lipid phosphatase activity, can also function as a molecular adapter to regulate midbody abscission during mitotic cytokinesis (PubMed:25659891). Can also negatively regulate TGF-beta and BMP signaling through Smad proteins dephosphorylation and retention in endosomes (PubMed:20061380, PubMed:23150675). {ECO:0000269|PubMed:11302699, ECO:0000269|PubMed:16787938, ECO:0000269|PubMed:20061380, ECO:0000269|PubMed:20736309, ECO:0000269|PubMed:23150675, ECO:0000269|PubMed:25659891, ECO:0000269|PubMed:27625994, ECO:0000269|PubMed:29962048, ECO:0000269|PubMed:30944173, ECO:0000269|PubMed:31543504}.