Hum, I think I found something. In Mail go to the 'Window' Menu, there you can see the Menu Item 'Previous Recipients', open it, now you can add all those items to your Address Book so in the future you have them available. I have found the file that has all those Previous Recipients, I hope you can find it under 10.3.9, it is placed here:
your Home folder -> Library -> Application Support -> Address Book -> "AddressBook.data.previous"
Check if you have that file under 10.3.9 and place it under 10.4. Well, maybe you could copy that Address Book folder and get Mail finally working for you.