What can I do if I'm only able to backup 4G of iTunes Library?

If you are only able to backup 4G of your iTunes library, please check whether the file system of the backup drive is FAT32. Click the drive and check it on bottom. If the file system is FAT32 it is only able to have a file size up to 4G.

To fix it, you need to firstly change your FAT32 file system into NTFS which has no limits and backup again to get the full file for restoring.

Follow steps below to change FAT32 file system to NTFS.
1. Open the Start menu and click All Programs.
2. Click the Accessories folder and right-click the Command Prompt shortcut. Select the Run as administrator option.
In Windows XP, click Run in the Start menu and type cmd.exe and press Enter.

3. Type convert F: /fs:ntfs and press Enter. Replace F: with the letter of the drive you want to convert.