Your post does not make sense according to what you wrote.
If you have an additional $2,000 of income that in no way would generate $4,000 of taxes.
Also, if your only income was just a total of $9,000 and you were not being claimed as a dependent then the first $10,300 of income would not be taxed assuming that you are filing as Single and all the income was from wages reported on W-2's.
You need to call the IRS using the phone number on the notice you received and find out what exactly they have for your income and the reason for the taxes owed.
