The difference between three-dimensional space (3D) and four-dimensional space (4D) is a challenging topic to define. Comparing the differences between something that is two-dimensional (2D) and 3D is as simple as comparing a flat drawing of an object and the object itself. Comparing 3D to 4D, however, is a much more complex topic.
What is 3D?
Whereas a two-dimensional (2D) view looks at objects in flat space, three-dimensional space looks at the objects in the way that we perceive them in open space. Unlike 2D space, it factors in things such a volume, depth, and shadowing. Simply put, three-dimensional space is the way we see and feel things from our perspectives.
What is 4D?
Four-dimensional space looks adds the factor of time and motion to the 3D object. It's not the time and motion we normally perceive in a 3D object, but a time and motion strictly associated with the object. Also known as hyperspace, four-dimensional space is a hard topic to define because it is something that we cannot visually understand. Because of this, 4D space exists only as a theoretical dimension that is relatively unknown by most people.
The Cube in 1D, 2D, 3D, and 4D
If a cube were put through dimensional space, it would appear as a dot in the one dimension, a square in two-dimensional space, and a cube in three dimensional space. In four-dimensional space, it is hypothesised that the cube would move in all directions (up, down, right left, side-to-side) at the same time.
It is often hypothesised that ghosts are a person's neurological remnants after he dies. Since these neurological remnants have no organs to go to, they exist in four-dimensional space in the sense that all of the ghost's senses move in multiple directions. A ghost's senses are said to be juggled and intermingled in different positions. According to the band MGMT, in their song "4th Dimensional Transition," we are going to turn into ghosts if the world ends in 2012. In this sense, the 4th Dimensional Transition refers to a change to an unstructured time.
The idea of four dimensional space is a concept that has baffled scientists for years. A professor at MIT may not even be able to offer a definition that an ordinary person could understand. It is perfectly normal for anyone to be confused by this subject.