You could just use 4 tracks set to 'record disabled (input monitoring only)' as input tracks and send them all to a 5th track that is set to 'record: output'. That track will record the mix of your 4 sources. Depending on the chosen record output mode, you can have a mono, stereo or a multichannel mix (the latter needs some additional settings concerning track channels). Come back with questions if that was not detailed enough.