The W-4 is the form that sets your paycheck withholding. You fill it out when you start a job and can update it any time. It asks for your filing status, whether you hold more than one job or your spouse works, dependents, other income, deductions, and an optional extra amount to withhold each period. Your employer feeds those answers into IRS tables and takes out the resulting federal income tax.
The form does one thing well: it withholds correctly on regular salary for someone with one job and no unusual income. It does less well when income comes in pieces. Bonuses and stock vests are withheld at the flat supplemental rate regardless of the W-4, and two-income households often under-withhold because each employer assumes it is the only paycheck.
The fix is Step 4(c), extra withholding. Suppose you expect $80,000 of RSU vests this year and your marginal rate is 35%. The 22% supplemental withholding leaves a gap of about $10,400. Dividing that across 24 remaining paychecks means adding about $433 per check on line 4(c), and the gap disappears without any quarterly payments.
The common mistake is filing a W-4 once and never revisiting it after a raise, a marriage, a second income, or a new equity grant. A five-minute update in January, or right after a big vest, prevents an April surprise.