Apple no longer signing the 4.0 or 4.0.1 ipsw, you can only restore to 4.0.2 ipsw. 4.0.2 is not jailbreakable at the moment and the Devteam will not work on it.

However, if you have saved your iphone shsh blob for the lower firmware (3.1.2, 3.1.3, 4.0 or 4.0.1) previously with saurik server or using tinyumbrella you will be able to restore to the lower firmware by redirecting the host of your computer to 74.208.105.171 gs.apple.com.

Only 3gs iphone with old bootrom can be jailbroken at 4.0.2 using the latest redsnow or Custom Restore ipsw.

please make sure you are restoring to the correct ipsw. For iphone 3gs the firmware file should start with iPhone2,1_ ...
the ipsw should look like the following
4.0.1 (3GS): iPhone2,1_4.0.1_8A306_Restore.ipsw
4.0.2 (3GS): iPhone2,1_4.0.2_8A400_Restore.ipsw