A Fourth Dimensional Being is a being that resides in a "fourth dimension" in a science fictional sense. This idea of a "fourth dimension" is somewhat based on the scientific concept of dimensions. This idea of dimensions is an attempt to give the Slender Man a pseudo-scientific basis for his movement abilities.
The basic overall concept for the idea of the fourth dimensional theory is based on the concept of extra spatial dimensions. The understanding of the theory is based on the conceptual idea of spacial occupation and movement. It is understood that humans can move and see in three dimensions, that is length,heighth, and depth, which, for this example, will be respectively known as X, Y, and Z. In Slenderman's case, he is capable of moving in X, Y, and Z, and in an undefined and unknown variable simply known as the mathematical variable "n" (to denote a lack of understanding of the required mathematical proof).
To explain this example, a hypothetical situation will be used in the idea of an indivdual forced to move in the second dimension is attempting to escape or capture a being moving in three dimensions. Due to the 2nd dimension being X and Y, the third dimensional individual has an advantage in being able to move in Z. Since the 2nd dimension individual is incapable of moving or utilizing the Z coordinate in his own plane, the 3rd dimensional individual does, therefore, have a distinct and serious advantage in both evading and capturing the 2nd dimensional individual. It would also make the third dimensional individual appear to vanish when moving in Z until they were once again directly in sync with the 2nd dimensions location on the 3rd dimension. It is also important to note that the 2nd dimensional individual CAN move in Z, but does not know how to as this individual cannot comprehend it.
Now it is extremely important to note that humans do not exist separate from the fourth dimension, but rather congruently with it, but are otherwise incapable of recognizing nor explaining it. While humans do regularly interact with "n", all one can physically perceive is X, Y, and Z.
The idea in this theory is that Slender Man is actually fully capable of seeing and freely utilizing "n" of the fourth dimension to his advantage. However it works (as this is all theoretical along with all other concepts), it gives Slenderman the appearance of "teleporting" (which is the ability to move from one location in space/time to another in space/time without going backwards in time and doing so instantaneously). This would mean, in layman's terms, that Slenderman is not teleporting whatsoever, but simply able to move and walk in ways we cannot understand, and since humans understand movement in X, Y, and Z, but cannot perceive, and therefore, take advantage of "n", Slenderman has a distinct and serious advantage.
Now since it is not factually understood, Slenderman could be utilizing the Fifth Dimension, Sixth, Seventh, or so forth, but it is simply assumed to be the Fourth as we cannot perceive properly yet.
It is important to note that while a Fourth Dimension does theoretically exist in time, it is NOT a spatial dimension, but rather a temporal dimension, and therefore this example is NOT Slenderman moving through time.