This is most commonly used to describe Korean celebrities with a strong personality. It may also be used to indicate that a person has a very strong individuality or a unique personality.
This is derived from ordinary people being "3d", so a person which is so quirky and thinks outside the box is considered "4d" or 4-dimensional.
I knew my friend was 4D! She always introduces out-of-this-world topics.
Daebak! His 4dness is starting to rub on me!