Very nice simulation you've got there! I also did a triple pendulum in JS a while ago. Quite a lot more than 100 lines of code, though. The equations of motion alone take up lots and again lots of space. (Imagine typing them in! I don't even dare to think about adding a fourth degree of freedom) http://users.jyu.fi/~samakapa/triplependulum/
My dynamics course teacher actually showed that the double pendulum can be linearized by some approximations (throwing out some negligible terms, small angle approximations etc.), and that way two modes of oscillation could be found: antisymmetric and symmetric oscillation. Just try with small initial angles with OP's script and you'll see.