An example is your pic, that is 4-d true physical apearence. At least thats what sony calls it.
4-D has nothing to do with photorealism.....and Wotja's sig is a 2-D image.
back when we had 2-D games what that meant was there were only 2 axis in the game that you could travel on....one axis (the y axis) was up and down...and the other axis (the x axis) was left and right. Now when we got 3-D graphics they added a new dimension (hence the "D") the new axis (the z axis) was depth so you could move deeper into the scene or closer to the screen.
Now we as people live in a 4-D world. What this means is that we can move up/down (y axis), side to side (x axis) and forward and back (z axis)....now the X, Y, and Z axis are all spacial dimensions the fourth dimension we live in is time.
Now if you wanna go really deep with it we get into string theory and M-Theory which is a unification of the 5 major string theories and ultimately brings us to a total 11 Dimensions (if you subscribe to M-theory), of which we as people only percieve 4. But to truly explain any of that I'm going to need you to at least have a basic understanding of differential equations and probably a solid grasp on general relativity and probably special relativity.