The W4P is not sent to the IRS. It's used by the administrator of your pension or annuity to determine how much tax to withhold from each distribution you receive during the tax year. If you'll take a look at https://www.irs.gov/pub/irs-pdf/fw4p.pdf General information about the form and it's purpose starts on page 2. Specific instructions for filing out the form start on page 3.
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