Why would you try blacksnow on 4.0
Make sure blacksnow is removed. That doesnt work on 4.0 baseband or firmware.
You're on the correct repo thru cydia?
Open Cydia
Check Sources, you must have added the UltraSnow repo ( repo666.ultrasn0w.com )
Search ultrasn0w, ensure that is the 0.93 version
Install it
After the installation, you need to reboot your iPhone