Hmmmm.. but to install a door/window sensor, you have to take out the batteries? Don't you need to then re-register the sensor?
Not if it's less than 4 hours. And if you do, the DS7000 "remembers" the sensor so that if you slide the switch to "Run-2" all you have to do is touch the "test" button on the sensor to re-register it back into the same position.
Once you take the batteries out of a DS10A it loses it's memory (and it's security code). More often than not, as has been well documented here, when you do reinstall/replace the batteries you have to hold the test button down for 10 seconds for the DS10A to resest and become usable again. When this happens, a new security code is selected internally and therefore you have to reinstall it in the DS7000 & OnAlert.
Sometimes you get luck and the DS10A keeps it's memory if you can remove & replace the batteries quick enough. But I don't think you could remove the batteries, mount a DS10A via screws, then reinstall the batteries in a small enough time for it to not lose it's memory.
The 4 hours you mentioned is just the time frame of how long the DS7000 will wait without hearing from the DS10A before it flags it as a problem.