The atomic subsystem is executed at 4s, 8s, 12s, etc. if enabled.
It is not "after it is enabled (regard this as time 0), then execut at time after 4s, 8s, 12s, etc" as you probably imagined.
Change the threshold value for enable flag to be 1,2,3 and see the effect.
To get what you want, you need to have some time related blocks inside the enabled subsystem. For example, a Pulse generator with period of 4 and pulse width 25%, to enable the atomic subsystem. You will get the desired output, once you set the threshold value to be 13 seconds.