4px

4px refers to a length measurement consisting of four pixels in digital graphics and user interfaces. A pixel is the smallest addressable element of a raster image. In web design and graphics, 4px is a common value for borders, gaps, strokes, and other line-based features.

In CSS, px denotes a CSS pixel, an abstract unit that is mapped to device pixels by the display's device pixel ratio (DPR). As a result, 4px does not always correspond to four physical screen dots: on a standard 1x display it may be four device pixels, while on a 2x or higher-DPR display it may map to eight or more device pixels. Browsers scale pixels automatically to maintain visual consistency across devices and zoom levels.

Common uses include border: 4px solid; padding: 4px; margin: 4px. Very small font sizes, such as 4px, are usually impractical for body text due to legibility and accessibility concerns.

Because px units are device-independent, the perceived size of 4px can still vary with screen density and user zoom. Designers often consider contrast, legibility, and layout when choosing a 4px value to ensure clear and accessible interfaces.

See also: CSS pixel, device pixel ratio, stroke width.