The restriction of having one tablet for one Wii U is a bit disheartening, but it makes sense. The controller is a dumb terminal. It is the Wii U that has to compute an image and stream it to the controller in real time. To support four tablet controllers, the Wii U would need four times the bandwidth and significantly more processing power. A console can theoretically support a near infinity of normal controllers, but this one is so expensive and requires the console to commit such an amount of resources that they might not be able to afford supporting even one more.