Is there an external battery available that is compatible with iPhone 4s and 5s? The company I work for is developing an app that uses geolocation and drains battery very quickly which is obviously a problem.

The customers will have both iPhone 4 and 5s so it would be great if such an external battery exists
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