4-butylresorcinol is a brightening resorcinol derivative also known as rucinol. It’s a potent tyrosinase inhibitor that exploits synergies with vitamin C to dramatically improve the appearance of hyperpigmentation—especially melasma.
Since it’s a rather new active in the cosmetic space, the amount of research available on 4-butylresorcinol isn’t as robust as the common brighteners we know and love—such as ascorbic acid and alpha arbutin. That said, there are some promising studies showing rapid results in visibly evening skin tone which outperform other brighteners on the market; even its relatives: 4-hexylresorcinol and 4-phenylethylresorcinol.1 2 3
 Huh, S., Shin, J., Na, J., Huh, C., Youn, S., & Park, K. (2010). The Efficacy and Safety of 4-n-butylresorcinol 0.1% Cream for the Treatment of Melasma: A Randomized Controlled Split-face Trial. Annals Of Dermatology, 22(1), 21. doi: 10.5021/ad.2010.22.1.21
 Shamanna, M., Mohan, M., Gowda, A., Kumar BC, S., Shree, S., Gangaboraiah, B., & Jaiswal, A. (2016). Assessment of efficacy, safety, and tolerability of 4-n-butylresorcinol 0.3% cream: an Indian multicentric study on melasma. Clinical, Cosmetic And Investigational Dermatology, 21. doi: 10.2147/ccid.s89451
 Kolbe, L., Mann, T., Gerwat, W., Batzer, J., Ahlheit, S., & Scherner, C. et al. (2012). 4-n-butylresorcinol, a highly effective tyrosinase inhibitor for the topical treatment of hyperpigmentation. Journal Of The European Academy Of Dermatology And Venereology, 27, 19-23. doi: 10.1111/jdv.12051