4-room HDB flats in Singapore measure between 90 to 110 sqm with three bedrooms, a living room, kitchen, and at least one bathroom. Types include 4STD (70-75 sqm), 4I (82-84 sqm), 4NG (92 sqm), 4S (84 sqm), and 4A (100-108 sqm). Popular among young couples and families for space and affordability.