That'll depend on your amp. For most 4 channel amps, you use channel 1 and 2 for your 2 speakers. To bridge channels 3 and 4, take pos from channel 3 and neg from channel 4 and attach it to your sub. Your manual should show you exactly which terminals to use. You might also want to check your amp's speaker connectors. Sometimes they have arrows or markers showing you which ones to use in bridge mode.