Form 4 (insider trade)

A Form 4 is the disclosure filing that company officers, directors, and shareholders who own more than 10% of a company's stock must submit to the SEC within two business days of any change in their beneficial ownership. It covers a range of transaction types, open-market purchases and sales, but also grants, option exercises, and shares surrendered for tax withholding, each identified by a specific transaction code on the form itself. Code P for an open-market purchase and code S for an open-market sale are the two most people mean when they talk about insider buying or selling in the legal, disclosed sense.

Read a Form 4 by transaction code first, then by size and context. An open-market purchase (code P) means the insider used their own money to buy shares on the same market anyone else trades on, generally treated as one of the more meaningful signals available, since it's a discretionary, personal-capital bet by someone with direct knowledge of the business. An open-market sale (code S) is far noisier, since insiders sell for all kinds of routine reasons (diversification, a home purchase, tax planning) that have nothing to do with their view on the stock.

A Form 4 doesn't tell you the insider's reasoning, and it doesn't distinguish a discretionary decision from a trade made under a pre-scheduled 10b5-1 plan set up months in advance specifically to avoid the appearance of trading on nonpublic information. The filing looks the same either way unless you check for the plan disclosure. It's also a lagging indicator by design. Two business days is the legal maximum, so the market has already had time to move between the actual trade and its filing becoming public. And a single Form 4, however large, is one person's decision, not a company-wide signal.

Say a company's CEO files a Form 4 showing a code-P open-market purchase of $1.2 million in shares, disclosed two days after the trade, with no accompanying 10b5-1 plan on file. That combination, discretionary, sizable, and using personal funds, is the profile generally treated as the highest-quality insider signal available. Now compare a Form 4 from the same executive showing a code-S sale of $400,000 filed the same week, executed under a 10b5-1 plan set up eight months earlier. Same insider, same company, but a scheduled diversification sale carries far less signal than an unscheduled purchase.