Together with PI4K2A and the type III PI4Ks (PIK4CA and PIK4CB) it contributes to the overall PI4-kinase activity of the cell (PubMed:11923287, PubMed:12324459). This contribution may be especially significant in plasma membrane, endosomal and Golgi compartments (PubMed:11923287, PubMed:12324459). The phosphorylation of phosphatidylinositol (PI) to PI4P is the first committed step in the generation of phosphatidylinositol 4,5-bisphosphate (PIP2), a precursor of the second messenger inositol 1,4,5-trisphosphate (InsP3) (PubMed:11923287, PubMed:12324459). Contributes to the production of InsP3 in stimulated cells and is likely to be involved in the regulation of vesicular trafficking. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + ATP = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol 4-phosphate) + ADP + H(+) Inhibited by phenylarsine oxide and adenosine (PubMed:11923287). Activation through membrane association is stimulated by active RAC1 (PubMed:12324459). Mainly cytosolic, association with membranes of the Golgi, endoplasmic and plasma membrane is stimulated by active RAC1 (PubMed:12324459). Association with early endosomes has not been confirmed (PubMed:11923287, PubMed:12324459). Widely expressed. Belongs to the PI3/PI4-kinase family. Type II PI4K subfamily.