What is the difference between a W-2 and a W-4? I know they're both US income tax documents I have trouble keeping them straight.
A W-4 is what you submit to your employer at the start of your employment or any time you have a need to update your information. It tells the company a little bit about your tax situation so that they can properly estimate how much tax needs to be withheld from your pay check.
A W-2 is what your company sends to you at the end of the tax year. It is a summary of all of the income you made from them as well as all of the tax that has been withheld from your paycheck for the year. This form can also include some other special cases such as showing things that were performed pre-tax.
The W-4 is the form with which you tell your employer how much tax to withhold. It's done with a very few bits of data. Marital status, number of exemptions (either dependents running around the house or deductions such as mortgage interest) each exemption is your way of telling the employer "don't tax $3800 of my income." Last, a box to say if you need more tax withheld.
W-2 is your employer's report to you at year end, everything about your earnings and withholdings, tax, child care, retirement, etc.
And to see how the employer converts this to a deduction from pay