Based on the instructions for the W-4P, you list your total expected income from your job or jobs on line 2(b)(i) and you list what you expect will be the taxable portion of social security, plus any other expected taxable income (interest, dividends, lottery prizes, etc.) on line 4a.
This information will be used by the pension payer, in conjunction with tables and formulas provided by the IRS, to calculate the appropriate withholding. If the pension is small and your other taxable income is modest, there might be little or no withholding, it depends on your overall situation. But the form is designed so that you list all your income as instructed, and let the formulas do the work.
