Is there any way I can design my classes for both iPhone4 and iPhone5 using the same xib?
If you want to make some small changes(e.g images position and sizes, scroll view content insets..), you can use codes to differentiate between iPhone 5 and iPhone 4/4s, and implement your codes inside the if/else statement.
Use this:
https://stackoverflow.com/a/12447113/1371949
to detect the iPhone type.