Who completes Form W-4
Every new employee completes Form W-4 at hire. The 2020 redesign eliminated the old "allowances" concept and instead asks for filing status, multiple-jobs adjustments, dependents, other income, and additional withholding in dollars. Employees may submit a new W-4 at any time to adjust withholding.
Why small-business owners care
If you have employees (including yourself if your business is taxed as an S-corp), the W-4 drives federal income tax withholding amounts that go on Form 941 (or 944) each quarter and on the employee's W-2 at year-end. Errors here mean either over-withholding (employee complaints) or under-withholding (employee tax surprises and possible penalties).
Self-employed alternative
Sole proprietors do not file Form W-4 for themselves — they pay quarterly estimates on Form 1040-ES. However, a self-employed individual whose spouse is a W-2 employee can use the spouse's W-4 (with extra withholding in Step 4(c)) as a tax-payment lever. Since withholding is treated as paid evenly throughout the year, increasing a spouse's withholding in Q4 can fix earlier estimated-tax shortfalls.
Multiple-jobs accuracy
Step 2 addresses the math problem of two earners or one earner with two jobs. Without it, each job withholds as if it were the only one, and the household ends up under-withheld. The IRS Tax Withholding Estimator at irs.gov is the easiest way to compute the right Step 4(a) and Step 4(c) entries.
Recordkeeping
Keep employee W-4 forms for at least four years after the date the tax becomes due or is paid, whichever is later. The IRS may request copies during a payroll audit.
Where the numbers actually flow
Every dollar that touches Form W-4, Employee's Withholding Certificate has a downstream destination — usually a line on Form 1040, a schedule that feeds Form 1040, or another supporting form. Tracing the flow once, with last year's return open in front of you, makes the form intuitive in a way that reading the instructions cold rarely does. The high-leverage takeaway is that small-business returns are interconnected: a change on one form ripples through three or four others, and a software package or preparer that does not recompute every dependent line on every change can produce silently incorrect results.