Delete the >=4 on Serial.available().
You should consider an input system that takes which joint to move and the value. If you enter 0,20(return key) you'd read the zero as the right servo and move it to 20 degrees. To figure out how to do this (taking in the values), see http://www.tigoe.com/pcomp/code/arduinowiring/1161/ for ideas.
Also when debugging, use some Serial.println("I'm at step x"); to let you know where the code is while it is running.