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We present nonlinear spectra of four-level ladder cesium atoms employing 6S1/2-4 6P3/2→ 7S1/2 → 30P3/2 scheme of a room temperature vapor cell.A coupling laser drives Rydberg transition,a dressing laser couples two intermediate levels,and a probe laser optically probes the nonlinear spectra via electromagnetically induced transparency(EIT).Non-linear spectra are detected as a function of coupling laser frequency.The observed spectra exhibit an enhanced absorption(EA)signal at coupling laser resonance to Rydberg transition and enhanced transmission(ET)signals at detunings to the transition.We define the enhanced absorption(transmission)strength,HEA(HET),and distance between two ET peaks,γET,to describe the spectral feature of the four-level atoms.The enhanced absorption signal HEA is found to have a maximum value when we vary the dressing laser Rabi frequency Ωd,corresponding Rabi frequency is defined as a separatrix point,ΩdSe.The values of ΩdSe and further η = Ω2dSe/Ωc are found to depend on the probe and coupling Rabi frequency but not the atomic density.Based on ΩdSe,the spectra can be separated into two regimes,weak and strong dressing ranges,Ωd ＜ΩdSe and Ωd ＞ ΩdSe,respectively.The spectroscopies display different features at these two regimes.A four-level theoret-ical model is developed that agrees well with the experimental results in terms of the probe-beam absorption behavior of Rabi frequency-dependent dressed states.