Whether the iPhone is "worth using" will depend on the user. Your initial question is in regard to the device requirements for iOS 8. Having said that, an iPhone 4S is not going to perform as fast as an iPhone 6 Plus running the same iOS version. With that in mind, if you're opting to get an iPhone, I recommend getting "at least" an iPhone 5S.
The 4s is still worth using, but not for long. A 5s would last much longer with the operating systems. But if you do get a 4s, read over these suggestions.
To help your 4s run as smooth as possible, try these suggestions.
1. Leave 20% or more of your internal storage space free. This helps to not constrain the system.
2. Get rid of unnecessary features. Yeah, some are cool, but they will bog down an older model phone.
3. Disable transparency effects. Toggle on "Increase Transparency" in Accessibility.
4. Toggle off "Background App Refresh".