Are you jailbroken on 4.3.1? If not then you probably dont have the option to save SHSH, & if the only SHSH you have for your device is for 4.3.5, then you can only restore to 4.3.5 in the future (say if the device gets updated to iOS5)
If your SHSH is saved for 4.3.1, then you CAN downgrade. Im sure you know the steps (if not, ask)
You can only restore to the firmware you have SHSH saved for. So if you have none for 4.3.1, 4.3.3, then you cant restore to either.