An N-type four level system where two coupling fields interact with two separate optical transitions was constructed in the present paper. By discussing the behaviors of probing field absorption profiles under the effect of different Rabi frequencies of two coupling fields, EIT, Mollow and Autler-Townes doublet can be seen and mutual transformation between them can be obtained. Multiple transition channels in the system were found and the results show that the system can be divided into several subsystems according to the transition channels. Quantum interference between different transition channels can be realized through different dividing methods, so three nonlinear effects with different generating conditions and physical nature can be seen in the system.