4MSK (INCI: Potassium 4-Methoxysalicylate) is a colorless to pale-yellow liquid known for its dual function in modern skincare: it acts as a potent skin brightening agent—inhibiting tyrosinase and melanin deposition—and also offers antibacterial/anti-fungal support, making it suitable for blemish-prone or acne-sensitive skin.

Soluble in both alcohol and oils, it blends easily into lotions, serums, and emulsions. Typical usage ranges from 0.1–0.5%, offering mild, low-irritation performance. Additionally, the enhanced “4MSK/Fluid Penetration” technology enables increased dermal uptake, improving brightening efficiency while maintaining skin barrier integrity.