Default.png size for iPhone 4?

Hi Guys,

I have an app out that covers the iphone and ipad, now I am making an exclusive iPhone 4 version of it to accommodate the crazy high res display of the iPhone 4.

Anyhow I am not clear on what to do with the deafult.png file, do I make it 640px x 960px to fit just right into the iPhone 4 screen or do I make it 320 x 480, since there is no way right not to limit a release to jus the iPhone 4, the default.png image has to cover all platforms.

Or is there is a way to denote the default screen is specifically for the iPhone 4 such as the iPad version of the deafult.png which is called default_portrait.png??

Thanks, let me know if there is a clear answer, could not find anything with the search.