Wearing the same clothes for 4 days is generally "OK" from a health perspective, but it depends heavily on the fabric type, activity level, and personal hygiene. Natural fibers like merino wool are famous for their antimicrobial properties and can easily be worn for 4 or more days without developing odors, making them a favorite for "one-bag" travelers and hikers in 2026. However, synthetic fabrics like polyester tend to trap bacteria and body oils, which can lead to noticeable smells and potential skin irritation, such as "folliculitis," if worn repeatedly without washing. If you are in a cool climate and performing low-intensity activities, rotating your base layers and allowing them to "air out" overnight can extend their wearability. For most people, the "social" limit is often reached before the "hygiene" limit; however, as long as you change your underwear and socks daily—which are the primary collectors of sweat and bacteria—wearing the same outer layers for 4 days is a sustainable and common practice for many modern minimalist travelers.