Part of the documentation you need to submit when filing bankruptcy is the last three years tax returns (not four), it requires you to provide the documentation not acutally file.
If you did not file the tax return for a specific year, you simply include an "Affidavit of No Tax Return Filed" , or similar document required by the Trustee for that particular year. State why you didn't file, if none was required then you are fine, and it shouldn't prevent you from filing bankruptcy.
However, you must be sure and file a tax return if you are filing Chapter 13, you will be required to file for all five years or face dismissal of the bankruptcy.
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