What does 4 s mean in iPhone?

4 means that it is the 4th generation of phone

There was:

1. Original iphone (2g data only)

2. iphone 3G (3g data)

3. iphone 3gs (better processor and camera)

4. iphone 4 (total redesign)

5. iphone 4s (a5 processor, dual radios, better camera, siri, etc)