>I am looking for a TV Tuner that can play 4 channels simultaneously. I guess someone already invented that. :)

Eh? :) For the PC, right?
This means basically that you need 4 separate tuners in one card or 4 separate cards. All I know of are double tuners for PictureInPicture TVs.
I wonder what will happen if you put into one PC 4 ATI TV-Wonder cards... :) ATI TV may show you all channel pictures at once, but only one little window at a time may be active.