The calculations used by payroll companies (which are specified in IRS instructions publication 15-T, if you are curious) are self-adjusting. If your taxable income goes down (because of increased 401k deferrals) then your tax withheld will also go down automatically. If you are happy with your current year-end tax situation (size of refund or amount of tax owed), that should be the same without changing your W-4, because if your income increases or decreases, your withholding will increase or decrease automatically.