A four-dimensional space or 4D space is a mathematical extension of the concept of three-dimensional or 3D space. Third dimension is the simplest possible generalization of the observation that one only needs three numbers, called dimensions, to describe the sizes or locations of objects in the everyday world. For example, the volume of a rectangular box is found by measuring its length (often labeled x), width (y), and depth (z). 

So 4D only imagination