4D alien is what the member V (Kim Taehyung) from Bts is sometimes called by
Armys (Bts fans) and other members. (Click for the meaning of
4D) And "alien" means that he's so strange people start to wonder if he's actually a human. But this nickname is not meant to be rude in any way, people just use it because they think he's cute.
However some people claim that V doesn't want people call him 4D alien because he doesn't like that people think he's so strange that they call him 4D or alien. But this is not confirmed.