100% downgrade Done 3GS 4.0.2 to 4.0.1
--------------------------------------------------------------------------------
. Go to C:\Windows\System32\drivers\etc
Open the host file with notpad
ADD THIS LINE AT THE END
74.208.105.171 gs.apple.com
after doing all these steps successfully
do these steps now carefully no need save shsh.
1-phone should be off now connect to itunes with usb it will turn on normal(phone should be activated).
2-now don't remove usb and turn off the phone normally.
3-now put the phone in dfu mode while connected.
4-now hold shift button and restore it with 4.0.1 it will be done successfully.
remember do these steps carefully as stated
best of luck