can iphone 4s update ios 8

Yes, it can, see:

http://mashable.com/2014/09/25/iphone-4s-ios8/

But if you upgrade to iOS8, you will no longer be able to use iPhoto iOS on your iPhone, only the new Photos.app will be available, that is much more basic.

The iPhone 4 can't update to iOS 8. Are you sure you have a 4S?

Identifying iPhone models