If you want to downgrade to 4.0.1, you need shsh blogs. What are you trying to do ?
You can downgrade to 3.1.2 or 3.1.3 without the need of shsh blogs by doing the Shift + Restore in Itunes and kick out of recovery from the feature of TU.
Ok, do this : Put your iphone into DFU mode and Shift + Restore to 3.1.2.
Error 1015 pops up
Kick out of recovery from the feature of Tiny Umbrella.
Now you are at emergency Screen, you can let your official sim to activate
Or Now run Redsn0w 0.9.4 to jailbreak it first.
Now from this jailbroken it, run ireb 4.1 and follow the instructios and then you will be asked to restore to custom firmware.
At this point do a Shift + Restore to Snowbreeze_4.1.ipsw which you have already created.
When building your cfw, make sure you unchecked the activate my iphone under Expert mode and under General.
It should now restore successfully.