Rotations

I'd need someone to explain me how the rotations work in a model. I know the basics about rotation types have been explaineed somewhere else but now what I need to know is the four numbers in a rotation ( { 1, 2, 3, 4 } ) are ? I've been trying to edit them but I often get strange results. For instance, if I wanted to transfer the rotation of a left arm to a right one, I'd need to "inverse" the horizontal movement. Can someone tell me how I can do it ? Thanks.

I had seen your tool in Oinkerwinkle's mdl tools thread, but for some stupid reason I had failed to understand how useful it was (I thought it was a tool to rotate geosets, a thing that his tools can already do). So thanks a thousand times, you don't know how much I appreciate this.