Indeed the fourth dimension is time.
This dimension is very significant, because it is used to prove Einsteins Special Relativity.
For instance, you must of heard that photons are ageless. Meaning, they have not aged since the beginning of time.
We know that we have x,y,z and the time dimension.
Also we know that x,y,z denote space coordinates.
Lets say that I am racing my car down an east to west track, where the finish line ends on the west side. Lets say that if my car travels at 100 km/h, it takes exactly 6 secs to reach the finish line. After several runs, we find something strange. The first 15 runs showed that the car indeed complete the track in 6 secs, but the other 5 times in 7,6.5 etc..
Lets say that the reason for this was, as I was racing down the track the sun was shinning, as a result of being partially distracted. So instead of travelling purely east>west, I was travelling North>South and partially east>west to arrive at the finish line.
Therefore if eastwest is considered to be one dimension and northsouth is considered the second dimension. Then my speed in the eastwest direction is not going to be 100km/h which explains the increased time. My speed is split between the eastwest and northsouth dimension. However if we add the speed in both dimensions we get that of light!
If all the speed were in the eastwest dimensional space then there would be no speed in the northsouth dimensional space.
Similarly, since a photon travels at the speed of light in space (i.e. x,y,z) then it has no speed left for the time time dimension, therefore the photon is ageless.
It is actually quite remarkable, the same idea is used to explain time dialation under constant speed.
If you have any questions let me know...I will be glad to help!
