What is a 4D?

Please don't yell at me, but exactly what is a 4d coaster, how does it work, and where is one?
Thanks!

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Rollercoasters are the secret of life!

http://www.woodencoaster.com

4D is a new Arrow Dynamics concept. The seats flips head-over-heels as the train navigates the track. The train rides on four rails instead of two. 2 rails acts normal; guiding the train, and the other 2 (which vary the distance from the guide rails) control the motion of the seats. Located at Six Flags Magic Mountain. The coaster is called X.

For good pics, go to www.twistedrails.com.

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A New World. A New Technology. One Last Hope for Salvation. Neon Genesis Evangelion