Cohors quarta Aquitanorum equitata civium Romanorum was a Roman auxiliary mixed infantry and cavalry regiment. It was probably originally raised in the Julio-Claudian era, perhaps under Augustus after the subjugation of the Aquitani in 26 BC. Alternatively, it may have been raised in a later levy of Aquitani after 14 AD. Unlike most Gauls, the Aquitani were not Celtic-speaking but spoke Aquitanian, a now extinct non Indo-European language closely related to Basque. Some tribes in Aquitania were Celtic-speaking however, such as the Bituriges.