Using a registry key to monitor security template changes

In case any other admins replace the security template, and I would know by using this unique registry key. Also by using this registry key, I would also know the number of desktops deployed with the desired security template.

Sure, it's possible to plunk in your own registry key using security templates, but I don't think your ultimate goal is going to be achieved. To me, it sounds like prevention is the most important thing you'll want to perform here. That is, the prevention of other admins stomping over the security template you lay down on those 4,000 desktops. In short, admins are admins, and registry key or not, there's nothing you can do really to stop a fellow admin if he's set on changing it.