Form 4 is the SEC-required disclosure of insider transactions in company securities — purchases, sales, option exercises, gifts, and other changes in beneficial ownership by officers, directors, and 10%-or-more shareholders. The form must be filed within two business days of the transaction (a deadline tightened by Sarbanes-Oxley Section 403, effective August 29, 2002 — prior to SOX, the deadline was 10 calendar days after month-end, which obscured the timing significantly). The two-business-day SOX deadline made Form 4 a high-frequency signal channel: insiders' transactions are publicly disclosed within days of execution, providing a near-real-time view of the most-informed parties' actions.

The classic empirical research on insider transactions — particularly Lakonishok-Lee 2001 ('Are Insider Trades Informative?', Review of Financial Studies) and successor work — established that insider purchases are systematically informative about future stock returns, while insider sales are substantially less informative due to liquidity-and-tax-driven motivations that are unrelated to the company's prospects.