@Pascal's comment on the OP's question is right. By simply adding the image, it removes the black borders and the app will use the full height.
You will need to make adjustments to any CGRects by determining that the device is using the bigger display. I.e. If you need something aligned to the bottom of the screen.
I am sure there is a built in method, but I haven't seen anything and a lot is still under NDA so the method we use in our apps is quite simply a global function. Add the following to your .pch file and then its a simple if( is4InchRetina() ) { ... } call to make adjustments to your CGRects etc.
static BOOL is4InchRetina()
{
if (![UIApplication sharedApplication].statusBarHidden && (int)[[UIScreen mainScreen] applicationFrame].size.height == 548 || [UIApplication sharedApplication].statusBarHidden && (int)[[UIScreen mainScreen] applicationFrame].size.height == 568)
return YES;
return NO;
}