Apple iPhone 4 does not recognize my DC car charger
The most likely explanation is that this is a generic charger not designed/tested for use with an iPhone. Have a look here for a more detailed explanation, but in a nutshell, iPhone-compatible chargers need to put out some very specific voltage levels on the USB data (NOT power) lines to be recognized by the iPhone.

So, unless this charger specifically states that it is iPhone-compatible, it just won't work without internal modifications, and you should just return it and get one that is made for iPhone.