In this paper we present experimental results on measuring the resolving power of a WFR mirror based on a degenerate four–photon interaction with a feedback loop in a photorefractive crystal. Experimentally obtained values of the resolving power of from 30 to 45 grooves/mm, under different conditions in the feedback loop well agree with independent data and theoretical calculations for a classical arrangement of the degenerate four–photon interaction. It is shown in the paper that a suppression of the generation of irreversed waves in the feedback loop provides for an increase of the resolving power. Influence of phase distortions of the incident beam on the resolving power is studied experimentally and good agreement with the theoretical estimates known from literature is obtained.