Currently Being ModeratedNov 11, 2012 4:27 PM (in response to walshreg)
The music has to be on the computer.
Copy everything from the windows computer to the mac and then sync the iphone.
The contents of the iphone will be erased and replaced with the selected content of the new computer.
Currently Being ModeratedNov 11, 2012 4:28 PM (in response to walshreg)
Transferring the library xml to another computer is not enough.
Transfer your iTunes library from the Windows laptop to the Mac per the instructions included with this link.
Which OS X version is the Mac running and which iOS version is your iPhone 4 running?