Is there any way I can design my classes for both iPhone4 and iPhone5 using the same xib?
Yes, you can use the same XIB to design for iPhone 4 and iPhone 5 by using Auto-Layout.
Build an app using iOS 6 as the Base SDK and use the Auto Layout feature to make screens that can scale for all type of screens. You'll need Xcode 4.5 to do this.
Get started with Auto Layout here:
http://www.raywenderlich.com/20881/beginning-auto-layout-part-1-of-2
http://www.raywenderlich.com/20897/beginning-auto-layout-part-2-of-2