What are the specifications for chargers for the 4S? Can it use a 5.5V? What are the input voltage limits? I have a 5.5V charger for another phone. It seems to me that if Apple has any design smarts that a 5.5V charger should work just fine.
I suggest you use a charger that is an Apple brand or specifies that it was developed for use on an iPhone. If you're like me this is a small investment to keep my iPhone working and under warranty.