The main difference lies in the intended use: CM-Guard CM4000 is a universal alarm detector for monitoring sensor statuses (e.g. door contacts), while PowerGuard CM4100 was specially developed for monitoring power failures. Both send notifications via app, SMS, call or email. If you want to monitor general sensors, the CM4000 is suitable. If you are primarily interested in mains voltage monitoring, the CM4100 is the better choice.