A four-level ladder atomic system, driven by two continued fields, is considered. We show that higher coherence and population transfer can be created between the ground state and the meddle excited state. The short wavelength signal can be obtained by stimulated Raman effect when the probe pulse is imployed on the high excited state under steady condition. We show the condition for maximum frequency conversion efficiency by numerical simulation and theoretical analysis, and investigate the influences of the intensity of the two coherent fields,the pulsewidth, the detuning and the various parameters of the atoms on the frequency conversion efficiency.