form includes the child tax credit option. Your employer will not make any assumptions as to your filing status, dependents, deductions, etc.

Just go down the W4 and put numbers in boxes. For you it sounds 4 total allowances. W4 assumes a standard deduction. If you will have child care expenses, home mortgage, real estate taxes, state income tax, etc and itemize then you will end up getting a big refund. If you have capital gains, interest income, etc, this will increase your tax bill.

Keep in mind that an exemption is worth 4,000 in income. For every $4,000 your taxable income is above or below your income minus the standard deduction you can adjust your withholding by one exemptions up or down. If you have itemized deductions worth more than 8000 on top of the standard deduction add two exemptions to your W4.

Also, you can change this after you see a couple of paychecks. Do an estimated tax return to include all of your expected income and deductions. If the tax being withheld won't cover it, reduce your exemptions. If it looks like you will get a large refund you can increase your exemptions.