PEG-4

PEG-4 is a low–molecular weight polyethylene glycol used mainly as a solvent/humectant and is typically included at low to moderate percentages in leave-on and rinse-off formulas. Clinical experience and patch-test data generally show low irritation and low sensitization rates, but its smaller size can sting on compromised barriers (eczema, post-procedure) more than higher-MW PEGs, and it can enhance penetration of other irritants in a routine. Given these real-world risks for highly reactive patients while remaining broadly well-tolerated, a gentle (0.3) score is the safest accurate commitment. Safety Notes: PEG-4 is commonly used as a low-level solubilizer/humectant and co-solvent in leave-on toners/serums and fragrances/actives solutions, where it can appear around ~0.05–2%. In rinse-off cleansers, micellar waters, and especially PEG-4–based solvent systems (e.g., some makeup removers, pre-shave/aftershave and hydroalcoholic or low-water solutions), it is used much higher as a primary vehicle/co-solvent, with consumer products observed in the ~20–60% range. PEG-4 has no specific EU/FDA maximum concentration limit as a cosmetic ingredient; practical upper levels are governed by skin feel, irritation potential, and the need to balance water/alcohol/oil phase solubility.