Originally Posted by Anorak
They're for bleeding the front and rear control circuits. The master cylinders on the handlebar and foot brake. So, yes there are four brake circuits for two brakes.
There are 6 circuits, you missed two of them on the servo. Each one goes like this: master cylinder to servo, servo to servo, then servo to caliper.
Flushing and bleeding the middle circuit for both the front and rear circuits keeps your servo from dying.