Phosphoinositide phosphatase which catalyzes the hydrolysis of phosphatidylinositol 4-phosphate (PtdIns(4)P) (PubMed:24209621, PubMed:27044890, PubMed:29461204, PubMed:30659099). Can also catalyze the hydrolysis of phosphatidylinositol 3-phosphate (PtdIns(3)P) and has low activity towards phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2) (By similarity). Shows a very robust PtdIns(4)P phosphatase activity when it binds PtdIns(4)P in a 'cis' configuration in the cellular environment, with much less activity seen when it binds PtdIns(4)P in 'trans' configuration (PubMed:29461204, PubMed:24209621, PubMed:30659099). PtdIns(4)P phosphatase activity (when it binds PtdIns(4)P in 'trans' configuration) is enhanced in the presence of PLEKHA3 (PubMed:30659099). a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol 4-phosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate Inhibited by bis-peroxovanadates and hydrogen peroxide. Interacts with TMEM39A (PubMed:31806350). Interacts with SEC23A and SEC24A; this interaction is reduced in the absence of TMEM39A (PubMed:31806350). Interacts with PLEKHA3 and VAPA and/or VAPB to form a ternary complex (PubMed:30659099). Trafficking between the ER and Golgi is regulated by nutrient status and by TMEM39A (PubMed:31806350). Localizes to endoplasmic reticulum-plasma membrane contact sites (EPCS) in the presence of phosphatidylinositol-4,5-bisphosphate (PubMed:27044890). Detected in heart, brain, lung, liver, kidney, pancreas and testis.