they charge me for a free app !!

Were you charged for the app or was it just a credit card holding charge ? If you had added or changed your credit card when 'buying' the app then a holding charge is likely to have been made to check that the card is valid and active, which should be removed from your card within a few days : http://support.apple.com/kb/HT3702

Or possibly an accidental in-app purchase

iTunes Store: About In-App Purchases

http://support.apple.com/kb/HT4009

Confirm with support and have them check your account and the app.

http://www.apple.com/support/itunes/contact/

What was the price they charged you?

I could be wrong, but aren't most authorization holding chargers just a single dollar? So I'm not sure the 4$ applies to that.

Thanks for keeping us posted here, because now Im intrigued as well.