I added 15$ of gift card and my balance went from that to 4$ I don’t even know what I got charged for ??

Log in to https://reportaproblem.apple.com to see all recent charges.

You probably had a pending purchase that was not able to be charged until you added the gift card.

Then ask Apple Support directly about the issue.

Get Support -

https://getsupport.apple.com/

Is it a subscription?

Managing auto-renewing subscriptions -

http://support.apple.com/kb/HT4098

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