Electric powered 4 wheeled skateboards are available.
However this skateboard style device has 2 wheels in the centre side by side and it balances itself using a gyroscope/accelerometer and inbuilt computer like the more well known “Segway” transporter.
This means it can balance while stationary, turn on the spot, as well as moving along and is steered by the rider using a Wii Nunchuck.
I also have a unicycle that uses the same principles.
Will bring 2 machines, one with exposed circuit boards and so on for education. I have photo album and flyers.
If you see title page 2/3 of the way down this website you can see several college groups have now built machines according to my on line instructions:
Read more about this maker at http://sites.google.com/site/onewheeledselfbalancing/