I heard that Siri can be installed on iPhone 4 but can't find it in the App Store? Is it because it is a non-approved app on iPhone 4 and why?   If so, is it only installable if it is a jailbroken iPhone 4?
The Siri app was withdrawn from sale to new people, but if you purchased it you can download it still from your history. It won't work without servers running, but you can still "get the app" from the store.

The app does nothing but check to see if the Siri servers are running. Since they are no longer operational - even people who have the app can launch it to see this announcement.