Based on the pertubation theory of quantum mechanics, the formula of ionizing cross-section of the four- photon ionization processes with either two- photon resonance three-photon near- resonance or three- photon resonance two- photon nearresonane is derived under the condition of coherent exciting ionization. The result implies that the ion intensities of the two processes above relate to the molecular population on the initial energy levels that the processes concern. The four-photon ionization spectra are experimentally measured and analyzed with this result, which shows that the ionization process is one with three-photon resonance.