4/3系统（Four Thirds System，又经常直称为四三系统）是一种常见的照相机感光组件规格，由奥林巴斯（Olympus）与柯达（Kodak）共同设计研发并推广。此规格针对数字单镜反射式相机，目的在于使不同制造商之间生产的交换镜头产品可以无差错的组合使用。4/3系统的名称由来是其影像感光组件的规格与过去曾经存在过的4/3吋直径摄像管相同（Video camera tube）。然而，这里的尺寸并不是影像感光组件实际的尺寸，而是成像圈的直径。大致上来说，影像感光组件必须位在成像圈正中央并且完全包络在内，因此其感光区域的对角线长度（22.5mm）远小于4/3吋（约33.87mm）。

The Four Thirds System is a standard created by Olympus and Eastman Kodak for digital single-lens reflex camera (DSLR) and mirrorless camera design and development.

The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from different manufacturers. U.S. Patent 6,910,814 seems to cover the standard. Proponents describe it as an open standard, but companies may only use it under a non-disclosure agreement.