I found a cool four digit seven segment display, but it only has 12 pins! bit of googling revealed that the seven segments of all four digits are commoned together, and each digit has its own common cathode. to make it useful, you have to use Persistence of Vision (POV) by cycling through the four numbers you wish to display and switching the cathodes on one at a time to show each of the four numbers, at a frequency higher than your eyes can detect. simple!
this example is not a real circuit; in reality the seven segment common cathode display has a real cathode on it. see this page for a real example: http://m.instructables.com/id/4-Digit-7-Segment-LED-Display-Arduino/
