X2 at Six Flags Magic Mountain was the first 4D coaster.
It would be nice to see some more built, but X was full of problems from what I remember reading and apparently is not the smoothest ride so parks might be hesitant to build one.
[video=youtube_share;0ZVhAqaEX_k]http://youtu.be/0ZVhAqaEX_k[/video]