What Are Insider Transactions (Form 4 Filings)?

An insider transaction is a trade in a company's own stock made by one of its officers, directors, or an owner of more than 10% of its shares, reported to the SEC on a Form 4. The filing itself, not a secondhand summary of it, is the primary record of exactly what happened.

How it works#

Section 16 of the Securities Exchange Act requires these insiders to report a change in their beneficial ownership within two business days. Each Form 4 lists the reporting person, their role, the transaction date, a one-letter transaction code, the number of shares, and, for an open-market trade, the price paid or received.

The transaction code is the single most useful field on the form. Code S is an open-market sale and code P a purchase; both reflect an active market decision. Code A marks new shares awarded as compensation and code F marks shares an employer withholds to cover the tax bill when equity vests, neither of which is a discretionary market trade at all, just standard payroll mechanics playing out in public filings. Code M, an option exercise, and code G, a gift of shares, round out the set most investors are likely to encounter, each carrying a different implication for what actually happened.

A Form 4 also states whether the reporting person holds shares directly or indirectly, the second case usually meaning through a trust, a spouse's account or another entity the insider controls. Indirect holdings can be large relative to the direct position reported on any single line, so the full picture of an insider's stake often requires reading several lines of the same filing together rather than any one row in isolation.

A real example#

Microsoft's chief financial officer sold 41,674 shares on 2026-09-14, executed across six separate price-tier lots between roughly $496 and $501 per share, transaction code S. The filing states the sale was made under a Rule 10b5-1 trading plan the executive had adopted back on 2026-06-10, a schedule set months before this particular trade took place.

A useful contrast sits in a Form 4 filed a few days later by Nvidia's chief financial officer. One line, dated 2026-09-16, carries code F for 40,747 shares, taxes withheld when a batch of equity vested, not a sale in any market sense. Three further lines dated 2026-09-17 carry code S for a combined 34,918 shares, sold under a separate 10b5-1 plan adopted on 2026-06-16. Reading the codes side by side is what separates the routine tax mechanic from the actual, scheduled sale on the very same filing.

What this means for an investor#

A pre-scheduled sale under a 10b5-1 plan is a weak signal on its own, since it was set in motion long before the trade date and says little about how the executive views the company today. Discretionary buying with an insider's own cash, done outside any pre-set plan and especially when several insiders do it around the same time, is generally read as the more informative case, because it is a voluntary bet with personal money rather than a scheduled diversification.

Cluster activity across several insiders in a short window, in either direction, is worth more attention than any single transaction, and the underlying filing always names whether a 10b5-1 plan governed the trade, which is the first thing worth checking before reading anything into a single sale.

Related terms#

Code A and code F entries on this same form are the equity-compensation mechanics that drive dilution over time, just recorded at the individual level rather than in aggregate. A primary source, read directly rather than through a paraphrase, is the only place the transaction code and the 10b5-1 disclosure show up in full. Telling a scheduled, routine sale apart from a genuine signal is exactly the kind of noise-versus-signal call this class of filing asks for every time.