4-hydroxy Diclofenac is a CYP2C9 metabolite of the NSAID diclofenac.[1] [2] [3] By inhibiting COX and subsequently suppressing PGE2 synthesis, it demonstrates anti-inflammatory and analgesic properties.[4]

Reference:
[1]. Shimamoto, J., Ieiri, I., Urae, A., et al. Lack of differences in diclofenac (a substrate for CYP2C9) pharmacokinetics in healthy volunteers with respect to the single CYP2C9*3 allele. European Journal of Clinical Pharmacology 56, 65-68 (2000).
[2]. Sawchuk, R.J., Maloney, J.A., Cartier, L.L., et al. Analysis of diclofenac and four of its metabolites in human urine by HPLC. Pharm. Res. 12(5), 756-762 (1995).
[3]. Godbillon, J., Gauron, S., and Metayer, J.P. High-performance liquid chromatographic determination of diclofenac and its monohydroxylated metabolities in biological fluids. Journal of Chromatography 27, 151-159 (1985).
[4]. Yamakazi, R., Kawai, S., Matsumoto, T., et al. Hydrolytic activity is essential for aceclofenac to inhibit cyclooxygenase in rheumatoid synovial cells. Journal of Pharmacology and Experimental Therapeutics 289(2), 676-681 (1999)