This paper addresses the Direction-of-Arrival (DOA) estimation problem for amplitude modulated signals whose number is more than that of the array sensors. The proposed method is based on an idea of virtual array. For source signals with amplitude modulation, such as binary phase shift keying (BPSK) and M-ary amplitude shift keying (M-ASK), we show that introducing in virtual array actually gives rise to processing the fourth-order moments of array output, which is related to higher-order statistics (HOS) techniques. While traditional HOS methods in array processing mainly exploit higher-order cumulants of the received data, we propose a DOA estimation method based on the fourth-order moments, which is of lower computational load than the fourth-order cumulants. Simulation results demonstrate the effectiveness of the proposed method for estimating DOAs of more source signals than array elements.