Can I use a 10 W ipad charger for my iphone 4?

I lost my iphone 4 charger a few weeks ago.
and i just bought an ipad charger not from an official apple retailer. (because they said it's ok but I don't really trust them)
It was pretty hard and rare to find an iphone charger in my country so without any thinking i just straightly bought it.
what I'm afraid is, my ipone 4 charge faster then it used too(with the 5W charger) and does it mean charging faster with a 10 W ipad charger is overpowering the battery and damaging it?

The key factors are the voltage and current output of the charger. An iPad charger would probably have a greater current output, and that's why the iPhone is charging faster. If the voltage output is the same as a proper iPhone charger, a higher current output won't damage the battery right away, but might do so in time.

I always use the ipad charger for my iphone 4. There's no any problem. If you are afraid damaging it, buy new iPhone battery charger.