Just set what you want to monitor, the every 4 hours is just how often it updates the server , if you get a high or low alert it will push to your device. It is always monitoring the level within the device. So If your level is high or low if you have the alert set correctly it will send you a message instantly.
They use to infact have the ability to never check, and you still get alerts. The checking is just so when you go to the app it shows the sensor status - battery level etc.
Make sense?