Here are the most popular ones on amazon:
uNu Power DX PLUS External Protective Battery Case for iPhone 4S & 4 (Matte Black/Silver, 2400mAh - Fit All Models iPhone 4 & 4S)
http://amzn.com/B0048IZL5E
Skiva - Protective Case and Extended Battery for iPhone 4 4S
http://amzn.com/B0096VY8QG
Alpatronix iPhone 4/4S Battery Case (Extended Battery) - Black (Also available in Pink, Blue/Black, and Blue/White)
http://amzn.com/B008WFKT7E
I have this next one for the iPhone 5 and it works great:
Maxboost Hybrid Detachable Battery Case for iPhone 4S & iPhone 4 - Black/Black (1900 mAh, Fits All Versions of iPhone 4 & 4S)
http://amzn.com/B00A2JVJ22