Is it correct to put down my IRA Contribution as a Tax Credit (Step 3) and Adjustment (Step 4) on my W-4?

[rjs]

No. A deductible IRA contribution is an adjustment. It is not a credit. Do not include it in Step 3. Step 4(b) tells you to use the Deductions Worksheet. Line 4 of the Deductions Worksheet specifically says to include deductible IRA contributions on that line.

It might be easier, and less error prone, to use the Tax Withholding Estimator on the IRS web site to figure out what to put on your W-4 form.