These phrases are over, and mostly incorrectly, used. So let us first give definitions of each that help to have an overview of the subject.
Firstly, the "D" in 3D and 4D refers to dimension. The dimensions are Length (how long something is), Breadth (how wide something is) and Depth (how deep it is) and Time (which encapsulates movement, animations and anything else that requires time to occur).
As such, a drawing that depicts the width, depth and breadth of an object, such as a cube, is 3D as it is showing the three dimensions. Being the clever creatures that we are we will typically only be physically seeing two dimensions (on a computer screen, or piece of paper), but are able to percieve the third dimension of depth.
So, if this "3D" image was printed onto a piece of paper it is depcited in a 2D medium. Paper only has two dimensions: left to right and up and down. So it is fair to say that a piece of paper is a 2D medium but, due to our cleverness, we can interpret lines to make a 3D shape.
Hopefully that makes sense, and we understand that 3D images are not actually all that clever, but merely a drawing that depicts three dimensions.
What might be a 3D image to us will probably be nothing more than a series of lines to a less intelligent creature, like a dog, pig or horse.
So what about 4D? You could argue that an image of a cube spinning around on a computer screen is 4D. It has length; it has breadth; it has width; and time is also used to create the impression of movement.
So we saw earlier that all 4D imagery is really just an animation that gives the impression of space. However, people (including those in our company) have used the term "4D Projection" to refer to mapped projections onto flat planes that give the impression of physical movement.
Before I try to explain this rather abstract concept, I suggest you first watch this video to see some basic techniques in use:
http://www.vimeo.com/10427551 This video was our first test of using these techniques in 2010 and we have subsequently done various projects for major international companies.
Hopefully this will amaze you and give the impression that the building is lit up from multiple angles using different light sources.
In reality the effects shown in this are all done with a single projector, staying in the same location throughout the demo.
The way this is done is by "mapping" the building in an animation program. This is first done by making a detailed 3D drawing of the building to work from.
See a screen shot of this being done here
Once this is in the software unlimited effects can be created. For example, we can create (in the computer) the impression of a ball bouncing between the pillars of the building, or water flowing down the building.
Once this is created, it can then be "rendered" (see wiki article on this) and then projected back on the building.
It is interesting to see what is actually being projected. See the link here so see what is actually coming out of the projector lens. On your computer screen it will seem surprisingly plain. But click here to see what it looks like in real-life. This was a product launch for Virgin Media.
You will see from this that the lack of light is just as important as the presence of light. In essence it creates a form of negative space that tricks our conditioned interpretation of the images going into our brains.
At this point I would like to pay homage to some of the finest 4D projections I have seen, from some incredibly talented visual artists, see below:
Inspirational, ground breaking and, once you figure it out, elegantly simple http://www.youtube.com/watch?v=Jzil8S7tUeY
Anti VJ - Arguably the pioneers of 4D mapped visuals - http://www.vimeo.com/14740982
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