The 4 minute 'sleep' means that it will not transmit any data over the mesh network to conserve power. The PIR sensor in the device is typically going to continue actively sensing the environment. The 'sleep' is conserving the extremely high level of power that is require to transmit data. Simply monitoring the PIR sensor requires very little power whatsoever. So, during the 4 minute 'sleep', the device is constantly 'watching' the PIR sensor's value. At the end of 4 minutes, if there has been no motion during that time, it will transmit a motion:inactive update. If there was any motion detected before 4 minutes was up, the 4 minute 'sleep' will have been restarted from the time of the last 'inactive' sensor value.
Hope this helps to explain what is going on.
