ClC-3, -4 and -5 are located mainly in intracellular vesicles and are thought to promote vesicular acidification. These proteins appear to function as electrogenic H+ /Cl- antiporters, similar to the prokaryotic ClC-ec1 isoform. Functional features of ClC-3, -4 and -5 are, however, very different from ClC-ec1. Macroscopic currents display a pronounced rectification and voltage-dependent gating, properties that are unusual for coupled transporters. To study transport properties of these channels in more detail, we performed whole-cell patch-clamp and intracellular pH measurements on HEK293 cells stably expressing human ClC-4. Stationary noise analysis of ClC-4 macroscopic currents revealed a Lorentzian power spectrum, in contrast to the expected single dispersion spectrum of coupled transporters and indicating predominant channel noise by ClC-4. Changes in the external anion composition alter the macroscopic current amplitude. Variance-current plots obtained from non-stationary noise analysis under external perfusion with different ions indicated that these changes are mainly caused by changes of the maximal open probability. In addition, BCECF fluorescence measurement of H+ flux show decreased H+ transport for anions that increase macroscopic currents e.g. enhanced channel-like behaviour of ClC-4. Our study demonstrates a functional duality of ClC-4 that can operate as channel as well as coupled transporter.