A "4-sphere" is a mathematical concept that extends the idea of a sphere into four dimensions. While a regular sphere is defined in three-dimensional space and consists of all points equidistant from a center point, a 4-sphere includes all points in four-dimensional space that are equidistant from a central point. This concept is often represented in higher mathematics and theoretical physics. In geometry, the 4-sphere is also known as a hypersphere. It can be difficult to visualize since humans perceive only three dimensions. However, mathematicians use equations and models to study its properties, which can have applications in fields like cosmology and string theory.