On any multi camera shoot all that is necessary to sync the audio and video occurs at the start of a recording use a “clapper board” or a loud hand clap.
Then when editing align the clap noise from each camera/recorder on the timelines and that will ensure your video and audio will be in sync. If IMovie does not allow you to work with multiple timelines then perhaps an economical option is to look at Adobe Premiere Elements as a start to edit your music. My first TV documentary in 2005 was edited with Elements. This YouTube video may help you, link is: https://www.youtube.com/watch?v=f_vRTbLn4lk
You can then delete the audio from the Zoom H4 recorder and select whatever video you want.
If you an amplifier to record your music best quality audio will come via a direct feed from your amplifier. If IMovie does not allow you to work with multiple timelines then perhaps an economical option is to look at Adobe Premiere Elements as a start to edit your music. My first TV documentary in 2005 was edited with Elements.
