I upgraded an app on my iPhone 4, but this new version doesn't work. How do I replace for the older version?
Do you have a backup? Time Machine or otherwise? If so you can restore the file from a backup.

The iPhone apps are located in ~/Music/iTunes/Mobile Applications and end with the ".ipa" extension.

It has been a while since I tried this but I think you just have to place the restored file in the proper folder and sync your phone.