Is it possible to downgrade iOS 9 to iOS 8 on 4S? Accidentally updated to 9 version from 7. It lags a lot so I want to downgrade it at least to 8 version. Is it possible?

Via standart restore feature i get “This device isn’t eligible for the requested build”
Since the signing window has closed for iOS 8, I don't think there are any ways for you to downgrade to iOS 8. If your device is on iOS 9.1, you're SOL for now. 

However, if you're on an earlier iOS 9 build, it is possible to downgrade to iOS 6.1.3 with Beehind or Odysseus, although you'll need to jailbreak your device in its current state to achieve this.