What is the fourth variable in the 4th dimension?

[Luongo]

is it time? what's your theory on the study of other dimensions? and applications from them?

[Nabeshin]

Almost without exception, when people talk about a 4D they mean spacetime, which has coordinates (t,x,y,z) for example. So yes, time would be the extra coordinate.

[Jacky817]

is it time? what's your theory on the study of other dimensions? and applications from them?

If I've interpreted your question correctly, the theory would probably be Special Relativity that deals with time-dilation and space time.

For applications, the GPS which makes use of satellites will need to use Eintein's equation in order for it to work accurately. (relative speed between a point on surface of earth and satellite's speed will cause time dilation which affects calculation of the coordinates)

Read up on the theory, very interesting and not that hard once you see the contrast between the terms "relative" and "absolute".

Sorry if I've misinterpretted your question haha =D