You can use many different types of inputs for the capacitive sensor. Among these are everything from Apples, salt water, and utensils (depending on the material). You can also use some of the different conductive materials from the Adafruit shop like Conductive Rubber, Woven Conductive fabric (try using this for wearable projects), or even conductive paint pens to turn any material into a conductive one.
Need more inputs? You can continue using Digital Pin 4 for your receiver pin and any other digital pin can be used to create a capacitive sensor.
To do this, just add a line in your code:
CapacitiveSensor cs_4_# = CapacitiveSensor(4,#);
You'll need to replace
# with the Digital Pin Number you want used as a sensor.
We're using Digital Pin 4 as the Receiver and Digital Pin 8 as the new sensor, so here's is how your breadboard should look: