barbosaa, looking at the picture of the SUB, it appears to be POWERED so, what you need to do is this:
1. Connect L + R signal from soundcard to WHITE & RED (L + R in) on rear of SUB
2. Connect the 4 channel amp to GREEN + GREY (L + R out) from rear of SUB to AMP
3. Connect the 4 Speakers to each AMP output
* if the amp is only a 2 channel amp, use both amps and get a phono splitter to split the L + R output from the SUB into two channels from the SUB's L + R outputs..
4. find a suitable power source for BOTH the SUB and the AMP(s)...as previously stated, it will probably need 10AMPS at 12volts, which is slightly more than your average PC PSU can output
i think...
5. You MAY also need to put 12v onto the 'head unit power on trigger', depending on the sub, it may power constantly without it...
Hopefully, I have read the schematic correctly, but I may be wrong...
For more help with this SUB, here's a link to a manual for it...
http://static.highspeedbackbone.net/pdf/JBL%20Marine%20MPS1000%20Manual.pdf