4D = adding time to 3D
5D = adding of costs, e.g. costs of materials
6D = adding things such as the project life-cycle
The differences between the different levels is really the amount of information present. You start with 3D and then add levels of information with each step.
About 1D each.
short answer is:
3D - Obvious, isn't it? volumetric Geometry
4D - Timing or phasing
5D - Costing / money
6D - Facilities Management / rooms info
Do a little search in google, you will be amazed with the amount of different definitions you can find!
Regards
Nando M