How to Read Form 4 Filings: A Practical Guide to Insider Trading Signals

When a corporate executive buys or sells shares of their own company, the SEC requires them to disclose the transaction within two business days. That disclosure is called a Form 4, and it is one of the most accessible windows into how the people running a company feel about its future.

But reading Form 4 data correctly requires understanding what you’re looking at — and, just as importantly, what you’re not looking at. This guide explains how Form 4 works, what the transaction codes mean, and how to use the data without falling into common traps.

What Is a Form 4?

A Form 4 is an SEC filing that reports changes in beneficial ownership of a company’s securities by corporate insiders. “Insiders” include:

• Officers — CEO, CFO, COO, CTO, and other C-suite executives
• Directors — members of the board of directors
• 10% owners — any individual or entity that owns more than 10% of a company’s shares

Every time one of these people buys, sells, receives, exercises, or gifts shares, they must file a Form 4 with the SEC. The filing must be submitted within two business days of the transaction.

Understanding Transaction Codes

Each Form 4 transaction has a single-letter code that tells you what type of transaction occurred. Here are the most common ones:

Code	Meaning	What It Signals
S	Open-market sale	Insider sold shares on the open market. This is the code most investors watch.
P	Open-market purchase	Insider bought shares with their own money. Often considered the strongest bullish signal.
M	Exercise of options/derivatives	Insider converted stock options or RSUs into actual shares. Not a buy — they already had the right to these shares.
A	Award or grant	Company granted shares as compensation. No cash changed hands.
F	Tax withholding	Shares surrendered to cover tax obligations on vesting RSUs. This is mandatory, not discretionary.
G	Gift	Insider gave shares to charity, family, or a trust. May be tax-motivated.
C	Conversion	Securities converted from one class to another (e.g., preferred to common).
D	Disposition to issuer	Shares returned to the company, often as part of a buyback or forfeiture.

For a deeper dive into every transaction code, see our guide to Form 4 transaction codes explained.

Which Transactions Matter Most?

Not all Form 4 activity is equally informative. Here’s a practical hierarchy:

High Signal

• P (open-market purchase) — When an executive spends personal cash to buy shares at market price, they are making an active bet that the stock is undervalued. Peter Lynch famously said: “Insiders might sell for many reasons, but they buy for only one — they think the price will rise.”
• Large S (open-market sale) outside of a 10b5-1 plan — A discretionary sale of a significant portion of holdings can indicate the insider believes the stock is fully valued or that risks are emerging.

Medium Signal

• M + S on the same day — Exercise-and-sell transactions often indicate an insider is monetizing vested options. The significance depends on how much they retain afterward.
• Coordinated selling — When multiple insiders at the same company sell in the same window, the pattern is more meaningful than any individual transaction.

Low Signal

• A (awards) — These are compensation events, not market-driven decisions.
• F (tax withholding) — Mandatory dispositions triggered by RSU vesting. The insider has no choice.
• G (gifts) — Often year-end tax planning. Charitable donations don’t indicate bearish sentiment.

Real Examples From 13F Insight

Let’s look at how these patterns play out with actual insiders you can track on our platform:

Systematic Selling: Laurence Fink (BlackRock CEO)

Fink has sold $848 million in BLK stock over his career through 729 transactions. His sales follow a semi-annual cadence: predictable windows in spring and summer, consistent lot sizes, prices across the range. This is textbook systematic selling — likely tied to a 10b5-1 plan — and is less informative about BLK’s prospects than a one-time discretionary sale would be.

Exercise-and-Sell: Michael Xie (Fortinet CTO)

Xie, who co-founded Fortinet, has sold $5.84 billion in career transactions. His recent Form 4 showed exercises (M code) of 648,570 shares at $16.90, immediately followed by sales (S code) of 343,106 shares at ~$81. The Form 4 shows zero directly-held shares afterward — but Xie still holds 33+ million shares through family trusts, as disclosed in the filing’s footnotes. This is a critical lesson: Form 4 “shares owned after” only shows directly-held shares, not the full picture.

CEO Selling After Record Results: George Kurtz (CrowdStrike CEO)

Kurtz sold $702 million in CRWD stock over 943 transactions. His most recent sales in February 2026 came days after CrowdStrike reported its best year ever ($5.25B ARR, 24% growth). The filings noted that sales were made to cover RSU tax withholdings — making them effectively mandatory, not discretionary.

How to Use Form 4 Data on 13F Insight

Here’s a step-by-step guide to tracking insider activity on our platform:

1. Find an insider — Navigate to any stock page (e.g., AAPL or NVDA) and look for the insider transactions section, or search for an insider by name.
2. Check transaction codes — Filter for S (sales) and P (purchases) to see market-driven activity. Ignore A, F, and G codes unless you’re specifically analyzing compensation.
3. Compare to career totals — A $5 million sale from someone who has sold $800 million is noise. A $5 million purchase from someone who has never bought before is signal.
4. Look at timing — Sales before earnings, after lockup expiry, or during known 10b5-1 windows are less meaningful than sales that break pattern.
5. Check shares owned after — An insider who sells 10% of their position is different from one who exits entirely. But remember: this number only reflects directly-held shares.

Common Misconceptions

“All insider selling is bad”

This is the biggest misconception. Executives receive a large portion of their compensation in stock. They have to sell some of it to pay taxes, diversify their portfolios, fund personal expenses, or make charitable donations. What matters is the pattern: is the selling systematic (on schedule), or does it break pattern (unusual timing or size)?

“Insider buying always means the stock will go up”

Open-market purchases are a positive signal, but insiders are not infallible. They have better information about the company than the average investor, but they can still be wrong about market conditions, macro trends, or competitive threats. Use insider buying as one input among many.

“Zero shares owned means they have no stake”

Form 4’s “shares owned after transaction” field only shows directly-held shares. Many executives hold significant stakes through trusts, LLCs, GRATs (grantor retained annuity trusts), and family holding companies. Always check the footnotes, and cross-reference Schedule 13D/G filings for beneficial ownership data.

“Small transactions don’t matter”

For most large-company CEOs, any purchase is significant because they already have concentrated exposure through their salary, bonuses, and equity awards. A $500,000 open-market purchase by a Fortune 500 CEO is a deliberate act — not pocket change.

Frequently Asked Questions

How quickly do Form 4 filings appear after a transaction?

Insiders must file within two business days of the transaction. Most filings appear on the SEC’s EDGAR system within 24–48 hours. 13F Insight ingests these filings and makes them searchable on insider profile pages.

What is a 10b5-1 trading plan?

A Rule 10b5-1 plan is a pre-arranged trading program that allows insiders to set up automatic buy or sell orders in advance. Because the plan is established when the insider does not possess material non-public information, it provides a legal safe harbor against insider trading accusations. Transactions made under these plans are less informative because they were programmed weeks or months earlier.

Can I see whether a sale was part of a 10b5-1 plan?

Yes — Form 4 filings include a checkbox indicating whether the transaction was made pursuant to a 10b5-1 plan. However, many filings do not check this box even when a plan exists, so the absence of the checkbox is not definitive.

What is the difference between Form 3, Form 4, and Form 5?

Form 3 is the initial statement of beneficial ownership filed when someone first becomes an insider. Form 4 reports changes in ownership (the most useful for tracking activity). Form 5 is an annual report of any transactions that should have been filed on Form 4 but were not.

Should I copy insider trades?

Blindly copying insider trades is not a sound strategy. However, tracking insider activity can provide useful context for your own research. The most valuable signals come from clusters of insider buying across multiple executives at the same company, purchases that are large relative to the insider’s existing holdings, and buying that occurs during periods of stock weakness.