Yes, the 4 minutes ringed a bell, i immediately looked it up, the manual says the base station needs approx. 4 minutes to sync with the sensors, but only for the first time you set it up, or when you need to change the batteries. It doesn't say it syncs every 4 minutes on its own. Are you suggesting that it is? Because if that signal is complex as you say it is, than that's got to be it.
I only saw that the 4-mins signal is 3-4 times longer than the 30-secs signal, didn't examine it.
"Data stored for further processing" could mean only one thing, it is stored in temporary memory and used to calculate the average values of wind speed, wind chill and possibly everything else measured.
It's just my idea, I didn't read it anywhere. Don't know if there is onboard processing in sensor.