Is there an external battery available that is compatible with iPhone 4s and 5s?
Yes.  Most external batteries that are USB can be hooked up to both the 4S's 30-pin port and 5s's lightning port.  As long as one end of the connecting cable is USB and the other end is the corresponding type for the iPhone in question.  One example is:

http://www.mophie.com/shop/iphone-4s-4/powerstation-smart-phones-tablets