How do I add the phonetic contact fields to a contact on iOS 5 when using an iPhone 4?
I can confirm after testing that these fields are not available on the iPhone 4 in iOS5, although they are available on the iPhone 4S.

However, if you have your contacts synced with Address Book on the Mac, you can add the field to the contact card via the Add Field menu item under Contact, and the field is then sent to your iPhone on the next sync. I am not able to confirm if you can do this via the iCloud web interface directly.