4D, or fourth dimension, is essentially the same as 3D with the addition of time dimension. The physical object becomes unified with space and time in a 4D continuum. That is spacetime.
4D is usually referred back to the 4D Euclidean space, which in mathematics is the Euclidean plane and three-dimensional space of Euclidean geometry, as well as the generalisations of these notions to higher dimensions.
Click next to read about 4D in the cinema industry.