OK thanks for the link Neil.
So you are saying that a charger with a higher current rating than 650mA would actually damage the S4 charging circuit?

From your S4 recharging page which states:

The rating for the S4 charger is as follows:
Input: 240 VAC~ 60 Hz
Output: 12 VAC~ 650mA

Is there any significance to this 650mA rating in the original S4 charger, would it be safe to use an adapter with 12VAC~ 1000mA or 2000mA rating?

Thanks.