A 4x4 is a piece of lumber (wood) that is approximately 4 inches in both width and height. However, similar to other nominal dimensions, due to the milling and drying process, a nominal 4x4 board may actually measure slightly less, typically around 3.5 inches by 3.5 inches.

4x4s are commonly used in construction and woodworking for a variety of applications. They are often used for structural support in projects like building frames, fence posts, and other load-bearing elements. 4x4 lumber is known for its strength and stability, making it a popular choice for many outdoor and indoor construction projects.