If you have someone call you on your iPhone 4 and you would like to add that recent caller's number to an existing or new contact, here's how:
1. Tap on Recents.
2. Now tap on the Right arrow text to the number.
3. Next tap on Create New Contact. If you want to add the number to an existing contact the use the option Add to Existing Contact.
4. Now select a contact to add the number to.
5. Save your new number.
You number has now been added to the contact list.