To do this downgrade you must have restored your iPhone to 1.1.3 and then followed this tutorial to unlock:
How to Unlock Your iPhone With ZiPhone GUI for Windows
WARNING: THIS TUTORIAL HAS A HIGH LIKELIHOOD OF BRICKING YOUR IPHONE. ANY TINY MISTAKE OR PROBLEM CAN MEAN A BRICK FOR GOOD!
Why would you still want to do this? The 04.03.13 baseband can be unlocked on 3.9 BL. This means you can have Google Locater working. Also, all old secpacks will not validate on 4.6. The 4.6 Bootloader requires a secpack of a newer version to let you do things with the baseband; whereas, the 3.9 BL requires the same version OR newer.
Perform these steps with your iPhone on and connected to your computer.
Download ZiPhone from: here
Double click ZiPhone.zip on your desktop to extract the folder ZiPhone..
Press the Windows key and R at the same time to bring up the Run window. Type in cmd and press the Ok button to launch the Command Prompt.
Type the following command into the Command Prompt to navigate to the ZiPhone directory:
Execute the command ziphone to see all its possible uses.
The program will prompt you to put your iPhone into recovery mode. To do this please hold down both the Sleep/Wake and Power buttons until you see the Apple logo. Once the Apple logo appears release only the Sleep/Wake button until you see the iTunes logo and cable on the screen.
If you have a 4.6 BL iPhone you can now downgrade your bootloader! Just simply type in this command:
ziphone -b -v
The program will now search for zibri.dat. Then it will search for the iPhone. Once found it will tell you to wait for 2 minutes and 30 seconds.
After this time your iPhone will reboot and it will be bootloader downgraded to 3.9 with the 04.03.13_G baseband!
NOTE*: After completion its probably a good idea to install BSD Subsystem first then OpenSSH from Installer.app
NOTE*: If you are in an unsupported country YOU MUST patch AppSupport after unlocking or your dialer will keep crashing as will your SMS:
How to Add an Unsupported Country to AppSupport Using Windows
THANKS***: Big thanks go to Zibri for creating ZiPhone. You can link to his blog for updates: here