4D stands for four dimensional and it's a K-pop term used to describe idols with unique personalities that differentiate them from the rest of the stars. In other words, it means those who think, act, and feel differently than how "normal" people react to things; whether it be their answers, responses, etc. that deviate from the conventional way of thinking. Here are list of the idols who are known for their 4D personalities.
Kim Hyun Joong may be known for his pretty boy looks, superstardom, and dance moves, but aside from the cool characteristics that make up this singer, Hyun Joong is everyone's favorite 4D personality idol. He may be soft spoken but Hyun Joong is known for his aloof ways and harmless blunt comments. For example, he once told Girls Generation's Jessica that he found her sister, f(x)'s Krystal, prettier than she. He's also admitted to surfing the web and leaving mean comments about himself on forums, he randomly hates dragonflies, and he's picky about microphones. He also has a deep love for aliens or spaceships and is on the lookout for ET sightings. Amongst fans, he's known as the 4D prince.
JYJ's most popular member JaeJoong is not only one of the nation's most beloved idols, but he's also one of the most beloved 4D personalities in all of K-pop. He's known for his space-like/otherwordly mind and acts like he's on a completely different planet at times. Here's a photo below to show off JaeJoongs true 4D-ness:
Kim Heechul is eccentric to say the least and is often to described as being 4D. He's hugely popular for his out-of-this-world personality and fans have dubbed his quotes and behavior as "Heechulisms." He thoroughly enjoys speaking about his "beautiful" physical traits, isn't afraid to be honest, and has no shame when it comes to admitting that he's a huge fan of Wonder Girls's Sohee.