Scenalyzer does this.

Here's a quote from the Scenalyzer 4 user manual:
“My camera has 4–channel sound– how can I capture the sound of channels 3 and 4?”

Set File > Options > Capture Settings > audio capture settings > write other audio-channel to separate wav file to YES. For each captured avi file, ScLive creates a wav file with the sound for channels 3 and 4 under the same name. If the tape does not have 4-channel sound for a particular scene, ScLive does not create a wav file for this scene. Please note that only recordings in 32 kHz/ 12 Bit mode are capable of 4-channel audio channels.