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. 
My team and I, we are huge believers that to be successful we need to know our weaknesses. It is just clueless to consider yourself to be a smart, efficient and pleasant person until somebody else does. And it doesn't matter that we often have people whom we don't care - there are always some whom we listen to and respect. 
The clean restored iPhone stays for 32 mins of usage and 4 hr 47 mins of standby, and goes down to 95% only! That's more than 3 times. Not good. Next, going thru the jailbreak, not touching Cydia or any apps. iPhone stays for 17 min of usage and 3 hr 35 mins and down to 96% only. Basically, that's a sign that jailbreak indeed doesn't drain the battery. Next, installing all of the updates that Cydia wants you to. It took me overnight to measure it, but seeing 15 min of usage and 9 hr 33 mins of standby and down to 95% only is clearly a good indicator of no battery drain going on. The last step was to install MyWi 4.3.4. It comes also with Mobile Subtrate. After spending some time with it - 1 hr 17 mins of usage and 5 hr 45 mins standby - brought my phone down to 78%, which is definitely stands out of the line of "good battery usage". So, either Mobile Subtrate is draining the battery or MyWi didn't do a good job writing an extension to it, and now it eats all he juice. I'll keep you updated, but I'm already upset so much - the most important reason for me was free tethering - and now I feel that I'm paying too big premium for it. 
