The quantitatively minor phospholipid phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P-2] fulfills many cellular functions in the plasma membrane (PM), whereas its synthetic precursor, phosphatidylinositol 4-phosphate (PI4P), has no assigned PM roles apart from PI(4,5)P-2 synthesis. We used a combination of pharmacological and chemical genetic approaches to probe the function of PM PI4P, most of which was not required for the synthesis or functions of PI(4,5)P-2. However, depletion of both lipids was required to prevent PM targeting of proteins that interact with acidic lipids or activation of the transient receptor potential vanilloid 1 cation channel. Therefore, PI4P contributes to the pool of polyanionic lipids that define plasma membrane identity and to some functions previously attributed specifically to PI(4,5)P-2, which may be fulfilled by a more general polyanionic lipid requirement.