4D can mean a lot of things; it refers to a 4-D film which is a combination of 3-D films with effects that are of a physical nature. They are common in modern amusement parks where water, air and motion are used to add value added affects to the entertainment experience.
It can refer to the term fourth dimension which in its turn can mean a number of things including space time or the concept of hyper space, the concept of a fourth dimension related to space, an album by the band Hypocrisy, books by either Dr Yonggi Cho or Rudy Rucker, software, computer games developer, record company etc.
The train 4D was a prototype double level train in Australia. It (4D) is also the name of a popular public lottery in Malaysia and Singapore. It is also the code allotted by IATA to Air Sinai.