Insider Buying: How to Read Form 4 Filings

When the CEO of a company spends two million dollars of her own money on its stock, she has to tell the SEC about it within two business days. That disclosure is a Form 4, and it is one of the few pieces of market data where the people with the best information are legally required to show their hand almost in real time. Learning to read these filings well takes an afternoon. Learning which ones actually matter takes a little longer, and that is what this guide covers.

You can browse recent filings on the insider trading tracker, but the raw feed is noisy. Most transactions are routine compensation events that tell you nothing. The skill is in separating the handful of genuinely informative trades from the flood of scheduled sales and option exercises.

Who counts as an insider

Section 16 of the Securities Exchange Act of 1934 defines three groups of corporate insiders who must report their trades:

• Officers: the CEO, CFO, president, principal accounting officer, and other executives with policy-making roles.
• Directors: every member of the board, whether or not they work at the company day to day.
• Beneficial owners of more than 10 percent of any registered class of the company's equity. This is how large activist investors and founding families end up filing alongside management.

These are the legal insiders. The word has nothing to do with illegal insider trading here: Section 16 insiders are allowed to buy and sell their company's stock, provided they are not trading on material nonpublic information and they disclose each transaction on time. When someone first becomes an insider, they file a Form 3 listing their holdings. Every subsequent change in ownership goes on a Form 4.

The two-business-day clock

A Form 4 must be filed within two business days of the transaction. That deadline dates to the Sarbanes-Oxley Act of 2002; before then, insiders could wait until the tenth day of the following month, which in practice meant the market sometimes learned about a trade more than a month after it happened. Today the lag is short enough that Form 4 data is close to a live signal, which is a big part of why it is worth watching at all.

Compare that with the 13F filings hedge funds submit: those arrive up to 45 days after the end of each quarter, so a position disclosed in a 13F may have been opened four and a half months earlier. Form 4 is the timeliest ownership disclosure the SEC requires.

Transaction codes in plain English

Every line on a Form 4 carries a one-letter transaction code. A handful of them account for nearly everything you will see:

Code	What it means	Informative?
P	Open-market or private purchase of shares	Yes. The insider chose to spend their own cash.
S	Open-market or private sale of shares	Sometimes, but weakly. Sales have many innocent explanations.
M	Exercise or conversion of a derivative, usually stock options	Rarely. This is compensation being converted, not a market bet.
A	Grant or award of shares from the company	No. The insider did not decide anything.
F	Shares withheld to cover taxes or an option exercise price	No. A mechanical tax event.
G	Bona fide gift of shares	No. Often estate planning or charity.

The practical takeaway: code P is the signal. Grants, exercises, tax withholding, and gifts are plumbing. A filing full of M and F codes looks dramatic in dollar terms but usually means an executive's annual equity compensation is vesting on schedule.

Why buying matters more than selling

There is a deep asymmetry at the heart of insider data. An executive might sell stock to buy a house, pay a tax bill, fund a divorce settlement, diversify a portfolio that is dangerously concentrated in one company, or simply follow a prearranged selling plan set up years ago. None of those reasons implies anything about the business.

Buying is different. An insider who already earns a salary from the company, holds unvested equity in it, and depends on it for their career has every textbook reason to put new savings elsewhere. When they buy more stock on the open market anyway, there is essentially one explanation: they believe the price is going up. Peter Lynch made this point famously, and the logic has held up. Sales are ambiguous; purchases are a bet.

What the research says

Academic work backs up the intuition. Josef Lakonishok and Inmoo Lee studied decades of insider transactions and found that insider purchases were followed by above-market returns, while insider sales carried little predictive value. Two refinements from that body of research are worth keeping in mind:

• Officer purchases tend to be more informative than director purchases. Executives live inside the operations; outside board members see the business a few times a quarter.
• The effect is strongest in smaller companies. A mega-cap is covered by dozens of analysts, so an insider's information edge is thinner. In a small cap with little coverage, management may be nearly the only informed party.

None of this means every insider purchase works out. Insiders are frequently early, sometimes by a year or more, and they are often value-oriented buyers who step in while a stock is still falling. The research describes averages across thousands of trades, not a guarantee on any single one.

Cluster buying: when several insiders agree

One purchase can be idiosyncratic. Three purchases by three different insiders in the same month are much harder to dismiss. Cluster buying, meaning multiple distinct insiders buying within a short window, has historically been a stronger signal than a lone trade, because it suggests a shared view inside the company rather than one person's hunch or one person's spare cash. When you scan the activity feed, a company appearing repeatedly with different reporting names attached is worth a closer look than a single large filing.

Size relative to the buyer matters too. A CFO doubling her personal stake says more than a billionaire founder adding a rounding error to his.

10b5-1 plans: trades on autopilot

Rule 10b5-1 lets insiders set up a written trading plan in advance, when they have no material nonpublic information, and then let the trades execute automatically on a schedule. These plans exist mostly for selling: they give executives a legally safe way to diversify without timing decisions. The SEC tightened the rules in 2022, adding mandatory cooling-off periods before a new plan can trade and a checkbox on Form 4 itself that flags whether a transaction came from a 10b5-1 plan.

That checkbox is your friend. A sale executed under a plan adopted eight months ago tells you what the insider thought eight months ago, filtered through a formula. It carries very little current information, and you should discount it accordingly. Unplanned, discretionary trades are the ones that reflect a live decision.

Red herrings

A few patterns look bullish but usually are not:

• Token buys before bad news. A small purchase, small relative to the insider's wealth and salary, made shortly before a rough earnings report can be an inexpensive show of confidence. Judge the dollars against what the person earns, not against your own budget.
• Exercise-and-sell pairs. A code M exercise followed the same day by a code S sale of the same shares is an insider cashing a paycheck. Some data displays will show this as both a buy and a sell; it is neither, really.
• Buys under a company stock ownership requirement. Many boards require directors and officers to hold a multiple of their salary in stock. A new executive buying shares to meet that floor is complying with policy, not expressing a view.

Combining insider activity with 13F data

Insider filings and hedge fund 13Fs answer different questions from different vantage points. A fund manager sees the industry, the competitors, and the valuation from outside. An officer sees the order book, the hiring plan, and the quarter in progress from inside. When both groups are buying the same stock, two independent, well-informed constituencies have reached the same conclusion with real money.

A practical workflow: start with most bought stocks to see where prominent fund managers added last quarter, then check whether any of those companies also show recent open-market purchases by their own executives. The overlap list will usually be short. It will also be far more interesting than either dataset alone, because the 13F tells you smart outside money agrees and the Form 4 tells you the view is current, not 45 days stale.

Insider buying is a screen, not a verdict. It points you toward companies where informed people are optimistic, and it does so faster than almost any other disclosure. What you do after that, reading the filings, understanding the business, deciding whether the price makes sense, is still the actual work.