We have studied theoretically constructive interference via collision-aided radiative excitation in an open fourlevel system using a density matrix approach. The four-level system consists of a A-type three-level quantum-beat configuration driven by one laser field and a fourth level coupled by a vacuum mode. It is shown that through the incoherent process (collision), coherence between widely-separated doublets and subsequent constructive interference can be realized. We analyse the effects of the collisio-induced coherent and incoherent decay rates, laser intensity, and energy separation of the doublets on the interference.Meanwhile, the constructive interference between the two transition pathways 3P1/2-4D and 3P3/2-4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na2-Na system is experimentally observed. A good agreement between the theoretical and experimental results is obtained.