Channel-forming tight junction protein that mediates paracellular chloride transport in the kidney. Plays a critical role in the paracellular reabsorption of filtered chloride in the kidney collecting ducts. Claudins play a major role in tight junction-specific obliteration of the intercellular space, through calcium-independent cell-adhesion activity. Interacts with EPHA2; phosphorylates CLDN4 and may regulate tight junctions (By similarity). Directly interacts with TJP1/ZO-1, TJP2/ZO-2 and TJP3/ZO-3 (By similarity). Interacts with CLDN1 (By similarity). Interacts with CLDN8 (By similarity). CLDN4 is required for tight junction localization in the kidney. Phosphorylated. Phosphorylation by EPHA2 is stimulated by EFNA1 and alters interaction with TJP1 (By similarity). Belongs to the claudin family.