4 Pin Header

In order to use all 7 available I/O pins you have to solder the missing 4 pin header. It might be possible that your Arduino board does not have this header because it is a) too old or b) not an original Arduino. It might be possible that you don't have a 16u2 on your Arduino board at all, if its a cheap clone. The 4 pin header and also the ISP header might be flipped on non official boards.

This pin header can be used for example for the Arduino as ISP as SS pin or for additional I/O usage. See the 16u2 pinout to know where the pins are located. PB1 is equal to pin 1 and so on.

First you need a 2x2 pin header from a shop of your choice. Then solder it like this: