Question: 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?

Answer: 
The Siri app was withdrawn from sale to new people, but if you purchased it you can download it still from your history.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.

![Retrieval Failed](https://i.stack.imgur.com/x04G6.png)
The previous free Siri app for iOS was removed from the App store sometime around Oct 4. (A developer can remove an app of theirs from sale at any time, and Apple purchased the developer of this app.) This app (for people who downloaded it before its removal from the App store) itself announces that it will stop working on Oct 15, most likely because the old servers for its messy protocol will be taken down. So the old Siri app will most likely stop working even if someone is able to install it.