What is ' 4 CHANNEL OUTPUT'? That looks like it has RCA outputs for left and right channels which may be normal line level outputs tied to volume control. If so, then use those connections to your subwoofer. The internal low pass filter on the subwoofer will handle making sure the right stuff is played back.
EDIT: Looking at the back of the receiver, and it looks like you can't use the 4 Channel Output connection for normal line level output. So, you likely will need a subwoofer which has speaker level inputs on it, or you will need a speaker level to line level audio converter.
There are a ton of these on Crutchfield's website. Typically needed for car radios which may not have RCA connections on them...
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