A quick follow up to http://live.prokhorenko.us/iphone-4-jailbreak-battery-drain 
What I tried. Restore to 4.0.1 (not 4.0.2 or you will loose a jailbreak). Jailbreak. Install only MyWi4 from Cydia with Rock, run MyWi4, get the first license, remove MyWi4, install Elpelle6's MyWi4 without Rock, remove Rock - now you've got pretty clean jailbroken iPhone4 with MyWi4 without Rock installed. Run MyWi4 - it will ask you to respring. I told it "Later" and probably that what helps. It does work anyways - I tried it. The battery runs out a little bit (I still believe MyWi4 is sorta dirty), but it's acceptable - under 5% overnight. Hopefully, it will work for you, too.
What I tried. Restore to 4.0.1 (not 4.0.2 or you will loose a jailbreak). Jailbreak. Install only MyWi4 from Cydia with Rock, run MyWi4, get the first license, remove MyWi4, install Elpelle6's MyWi4 without Rock, remove Rock - now you've got pretty clean jailbroken iPhone4 with MyWi4 without Rock installed. Run MyWi4 - it will ask you to respring. I told it "Later" and probably that what helps. It does work anyways - I tried it. The battery runs out a little bit (I still believe MyWi4 is sorta dirty), but it's acceptable - under 5% overnight. Hopefully, it will work for you, too. 