Congenital quadricuspid aortic valve (QAV) is a very rare valvular malformation. We report four cases of congenital QAV with moderate to severe aortic regurgitation. All cases were accidentally diagnosed by echocardiography, and none of the patients complained of any symptoms related to valvular dysfunction. Three of the four cases had QAV with an accessory cusp smaller than the other three cusps, and the four cusps were the same size in one case. There are some morphologic classifications of QAV, but the prognostic differences in the morphology of QAV have not been clarified. However, careful echocardiographic observation of QAV is important, because it has been reported that the surgical management of QAV is affected by the position of the accessory cusp.