The name "3D" in general means a Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameters) are required to determine the position of an element (i.e., point). This is the informal meaning of the term dimension.
What is 4D?
The addition of special effects to a theatrical presentation to create a "fourth dimension" - the sensation of tactile experience: scent, temperature, touch. Examples:Beetles tickle the legs of unsuspecting cinema-goers, the wing beat of a gigantic dragon breezes through the rows of seats and spiders descend from the ceiling.
Why do I need to wear special glasses to view 3D?
The reason you need to wear glasses to watch 3D is that a separate image needs to be sent to each eye, with the brain combining the two images into a single image with 3D characteristics. In other words, the 3D process actually fools your brain into thinking it is seeing a 3D image, so it creates one for you.