iOS8 v3 was not workable at all for me. Battery life was terrible and the phone was always very warm. Might get 2 hours of use before the battery died. Downgraded back to iOS7 and everything was fine.
Same problems in iOS8 v4. Maddening.
Frustrated, I set up iOS8 v4 as "new phone" on my 5s. Now I might make it through a day just barely if I don't use the phone very much and with little to no data use at all.
With iOS7 I never had a care that I wouldn't get through the day even with very heavy phone and data use. I never had to recharge during the day with iOS7. Never. Not once.
The other day on iOS8 v4, I had to recharge my 5s three times in one day and that wasn't even with really heavy use.