If you file a tax return you have to report all your income, no matter how little it is. You have to report the income whether or not you get a W-2.
You have to enter all your self-employment income on your tax return, even if it's less than $400. What you might have seen is that you do not have to file a tax return if your only income is self-employment income of less than $400. And the limit is $400, not $4,000.
