When referring to 4-D space, people often use "time" as the fourth dimension, but mathematics allows for a fourth spatial direction, according to Case Western Reserve University. Spatial four-dimensions are harder to visualize because 4-D is foreign to the human brain.
You can visualize 4-D space on paper using a four-dimensional cube called a "tesseract." To form a tesseract, take two 3-D cubes and connect their corners, according to John D. Norton of the University of Pittsburgh.
Even if you use time a the fourth dimension, it does not work like up, sideways and down, according to Charles Liu, an astrophysicist with the City University of New York, College of Staten Island. In space-time, you can only go forwards in time.