Squiggy - welcome to our forums.
The problem lies in the fact that time is the 4th dimension only in the 4D space-time which we perceive (3 spatial dimension plus 1 time dimension).
When we talk about more"spatial" dimensions, time is _not_ considered a dimension. So when you take a cube and expand it into the 4th dimension, it is not expanded into time but into an (for us)unseen dimension. That is why these extra dimensions are called "mathematical" or even "hypothetical". However, they are useful in string theory and in mathematics.
So time is _not_ the 4th dimension.
Check out our hypography on hyperspace at
http://www.hypography.com/topics/hyperspace.cfm
Tormod
