The 4S will never draw more than 5VDC at 1A so an iPad charger is no more effecive than any other Apple-spec 1A charger.
It's been measured but some people just can't get past the placebo effect.
If people feel better using an iPad charger it certainly won't hurt.
The flipside of this is that that the default charge rate of the iPhone is 500mA and it needs a charger that can tell the iPhone to switch to the higher charge rate.
The Apple spec calls for chargers to use the two USB data wires to do this.
So all 1A Apple-spec chargers are equal but even a 1A or 2.1A charger will still charge the iPhone at the 500mA rate if not built to Apple's specs.