4D exist out of 4 Dimensions. Each Dimension consists out of different zones, with its own quests, adventures and secrets, all consistent with the Time Period. To have a closer look at the different zones, just click on the dimension you're interested in, and you'll get a more detailed look of how the dimension is setup.
Next to those 4 Dimension there's also a Central part, where people can gather, and talk about things, or find information concerning other areas in the game.

The Central Area
 
The Prehistoric Dimension

The Medieval Dimension

The Old West Dimension

The Future Dimension