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The Siri app was withdrawn from sale to new people, but if you purchased it you can download it still from your history.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)
There are stories in the blogosphere that hackers have migrated Siri from the 4s OS code onto an iPhone 4. There was a video demonstrating it.

It's not for the masses. In fact, it had some substantial problems.

There was a story a day or two ago that Apple is working on releasing Siri for older phones (the story said iPhone 4 and maybe 3Gs). Then a story yesterday said that's not the case.

If you're Apple and you've just released a phone that is all new on the inside but looks the same on the outside, and has this one killer feature that revolutionizes everything... would you be in a hurry to put that feature on older phones? I wouldn't. You'd gut sales of your new device. You *might* bring that feature to older devices in maybe six months or something, but you'd for sure let it be a unique draw to your new device for a good while.