You'll have to be a lot more specific about your hardware. First off, I can tell you that commonly:
Green: Line out
Blue: Line in
Red: Mic in
So you have the input of your amp plugged into two of your computer's inputs. Both are expecting to hear sound from the other.
If your motherboard really does support 4 channel output then its possible, after changing some settings, to get output from either the blue or red jack - but we have no idea what kind of hardware you have.
The simple solution is to split the output from the green jack so it goes to all of your amp's inputs, as is shown in the first diagram HERE.