If you are required to file a tax return you must enter all taxable income from all sources on your tax return. If you only had $4000 of income and it was reported on a W-2, you are not required to file a tax return. If federal income tax was withheld, you should file a return to get a refund.
If your income was reported on a 1099-MISC or no form, then you must file a return to pay self employment taxes.
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