US: 4-6 to United Kingdom Size 16-18
Cross-market women’s sleepwear conversion mapping 4-6 specifications into 16-18 for nightgowns, pajama sets, and sleep shirts.

Analyzing Women’s Sleepwear Equivalence Across Markets
US: 4-6: Corresponds to a relaxed bust range of 84–88 (33.1–34.6 ), elastic waist range of 66–70 (26–27.6 ), and comfort drape classification Relaxed / Comfort Drape.
United Kingdom Size 16-18: Corresponds to a relaxed bust range of 97–101 (38.2–39.8 ), elastic waist range of 79–84 (31.1–33.1 ), and comfort drape classification Relaxed / Comfort Drape.