The 4th direction is taken as time. To understand this imagine every space we placed next to one another being associated with a moment of time, like images are associated with time in a video and series of these images when played make up a video.
In Physics we call these 4dimensions as spacetime. One element(space) of 4th dimension is exactly one moment in the universe, and series of this describe the whole timeline of events taking place from very beginning to the future of the universe.
:-)