Step 1:
Download
Hex editor.
Step 2:
On "Computer B", backup "iTunes Music Library.xml" and "iTunes Music Library " safely.
Step 3:
On "Computer A", open "iTunes Music Library.xml" with Notepad and search (Ctrl+F) for "<key>Library Persistent ID</key>".
Step 4:
Following "<key>Library Persistent ID</key>", copy the code between <string> and </string>.
Step 5:
Find the same code on Computer B, then replace it with the code from Computer A.
Step 6:
Open "iTunes Music Library" by using the Hex Editor on Computer B.
Step 7:
Go to Edit > Replace in the Hex Editor.
Step 8
Insert code from computer B in "Find", insert code from Computer A in "Replace With"
Step 9:
Save this file, then close it.
Step 10:
Open iTunes on Computer B, and plug in your iPhone.
Step 11:
Select "Manually manage music and videos".
Step 12:
Now, you will no longer be alerted to erase your data, and you could
sync your iPhone 4 to more than one computers now.
