Comment 4 for bug 1241721

 "4":
     agent-state-info: '(error: invalid URL "http://cloud-images.ubuntu.com/releases/streams/v1/index.sjson"
       not found)'
     instance-id: pending
     series: precise

That url resolves for me, but it's probably spurious. I suspect machine-4 just failed to provision due to EC2 instability, deleting it and retrying the rabbit deploy may be enough. Unfortunately the debug-log doesn't contain anything incriminating about the actual start instance failure.