4D refers to the fourth dimension of time when used together with the first three dimensions of space.
2D = flat. 3D = spatial. 4D = space/time physics.
It's a big thing to get your head round. In terms of design I believe it's to do with time, as a dimension. Design which perhaps changes with time or utilises time to purvey a message.
Design Examples:
Reflections:
I'm starting to get more and more interested in design which is breaks conventional format constraints. These 4D designs are extremely exciting.