Deductions & credits

Remember that changing the W-4 does not change your tax liability -- the amount the government takes from you over the year, after considering any payments, withholding and refunds.  If you have more money withheld from each paycheck, you will get a larger refund but you will have less take-home money in each paycheck to spend on weekly or monthly expenses.  A large refund is really just an interest-free loan to the IRS.