My top Dentist is still using his 4s and had similar problems. . . but iOS 8.1 and then 8.1.1 and 8.1.2 solved the vast majority of them on his iPhone. He is quite happy now.
Some people with the 4s had no problems at all. I think it may have had to do with parts from different vendors installed in various versions of the 4s. The iPhone 4s was the most sold iPhone in history and made in various assembly plants in China and in Brazil under various contracts. Over the time it was made, and because of the quantities, the suppliers for component chips varied over time. This may have made a difference on how particular iPhone 4ses responded to the upgrades.