Unity 4 Build not working on iOS 5

Hi, I've built a children book application using Unity 4.0 pro for iPad. It's working fine on my iPad 2 with iOS 6.1.2. But when I installed it on iPad 1 iOS 5.0 it crashed even before the app reach main menu. I thought the problem was memory problem, so I ask my friend to installed it on his iPad 2 iOS 5.0.1 and it suffered the same problem.. Can anyone help me find out where the problem is? FYI, in the build settings, in the Target IOS Version I put iOS 4.0. Thank you