Form 4 Transaction Codes Decoded: Signal vs. Noise

Two Form 4 filings can show identical dollar amounts and yet mean completely different things. One is a CEO’s conviction buy. The other is automatic tax withholding. A single letter tells you which is which.

Every transaction on a Form 4 filing carries a Form 4 transaction code—a single letter in column 3 of Table I or column 4 of Table II. Most investors skip right past it. That’s a mistake, because that code is doing the heaviest interpretive lift on the entire form. Only about 11% of all Form 4 filings represent actual open-market purchases—the rest are compensation mechanics, option exercises, and tax-withholding events that carry no directional signal at all.

This guide gives you a complete taxonomy of form 4 transaction codes, a framework for telling signal from noise, and a three-question decision tree you can run on any filing in under a minute.

Why a Single Letter Changes Everything

Picture two Form 4 filings, side by side. Both show a CFO at a mid-cap company. Both show 10,000 shares and a value around $250,000. The only difference is the transaction code: one reads P, the other reads F.

The P filing is the CFO writing a personal check to buy shares on the open market. He is putting his own money at risk because he believes the stock is undervalued. The F filing is the company automatically withholding shares to cover the CFO’s federal income tax when his restricted stock units vested that morning. He made no decision. He had no choice. The shares were withheld before he ever saw them.

The SEC itself notes that many investors believe reports of insiders’ purchases and sales “can provide useful information about insiders’ views of company performance or prospects.” That statement is true—but only for the right codes.

How Form 4 Transaction Codes Work: Table I vs Table II

A Form 4 has two transaction tables. Table Icovers non-derivative securities—common stock, preferred stock, and similar. Each row carries a transaction code in Column 3. Table IIcovers derivative securities—stock options, restricted stock units (RSUs), warrants, and convertible notes. Each derivative row carries a code in Column 4.

The official list of codes lives in Item 8 of the Form 4 General Instructions. There are roughly 20 codes in total, grouped into four broad categories: general transactions (P, S, V), Rule 16b-3 exempt transactions (A, D, F, I, M), derivative securities codes (C, E, H, O, X), and other codes (G, L, W, Z, J, K, U). The SEC’s bulk EDGAR dataset uses the same transactionCode field throughout.

The Only Two Codes That Matter: P and S

Code P — open-market or private purchase. The insider spent their own money to acquire shares at the prevailing market price. This is the most studied signal in all of insider-trading research. A 2026 arXiv study of 17,237 open-market purchase filings across 1,343 microcap issuers found that a gradient-boosting classifier trained solely on Form 4 data achieved an AUC of 0.70 on out-of-sample 2024 data, with distance from the 52-week high accounting for 36% of predictive power. Transactions disclosed after price appreciation exceeding 10% showed a mean cumulative abnormal return of 6.3%.

An Alpha Architect review of academic literature found that a long-short portfolio based on insider purchase and sale sequences earned monthly alphas of 1.71% (22.6% annualized) overall, and 2.37% monthly (32.5% annualized) for top executive trades. The underlying research consistently finds that open-market purchases outperform sales in predictive power.

Code S — open-market or private sale.The insider sold shares at market price. Unlike purchases, sales are noisy. People sell for diversification, taxes, a home purchase, estate planning—reasons that have nothing to do with their view of the stock. Academic research consistently shows sale signals carry far less predictive power than purchase signals. That said, large sales representing a meaningful percentage of the insider’s total holdings, and especially cluster selling by multiple insiders at the same company, do carry a stronger negative signal and are worth flagging.

The Noise Codes: F, A, and M

These three codes generate more investor confusion than all the others combined, because they appear constantly and their share amounts can be large. None of them represent a discretionary investment decision.

Code F — payment of tax liability by withholding securities. When restricted stock units vest, the IRS treats the newly acquired shares as ordinary income. The company withholds a portion of the shares to cover the tax bill before handing the rest to the insider. The insider never touched those withheld shares; the company retained them automatically. Code F is classified under Rule 16b-3 exempt transactions, confirming it carries zero market-direction signal.

Example:An executive’s RSUs vest and she receives 20,000 shares. The company immediately withholds 7,400 shares (at the current stock price) to satisfy federal and state withholding. Two rows appear on the Form 4: an M for 20,000 shares acquired, and an F for 7,400 shares disposed. Neither row reflects a trading decision.

Code A — grant, award, or other acquisition pursuant to Rule 16b-3. The company granted the insider stock, options, or RSUs as part of their compensation package. The insider did not pay for these shares. They did not choose to acquire them in a market transaction. They simply received compensation. Code A filings flood EDGAR during proxy season, when annual equity grants are made.

Example: A newly promoted VP receives 15,000 RSUs as part of her offer package. A Form 4 with Code A for 15,000 shares appears. This tells you she was promoted, not that she believes the stock will outperform.

Code M — exercise or conversion of a derivative security. The insider converted options, RSUs, or warrants into common stock. This is a compensation event, not a market transaction. Code M almost always appears alongside a Code F (tax withholding) and is often followed by a Code S (the insider selling the shares they just acquired to lock in cash or cover costs). The M-F-S sequence is the single most common pattern on Form 4 and carries no directional signal.

Derivative Exercise Codes: X, O, and C

These codes appear in Table II and describe what happened to a derivative security. None of them are bullish signals on their own.

Code X — exercise of in-the-money or at-the-money derivative.The insider exercised a stock option that was profitable (the stock price was above the strike price). This is almost always immediately followed by a Code S in Table I, as the insider sells the shares they just acquired to realize the gain. The exercise itself tells you nothing about the insider’s forward view; it tells you the option was worth exercising. Hunton Andrews Kurth’s Form 4 training guide confirms that X is the most common derivative code.

Code O — exercise of out-of-the-money derivative. The insider exercised a derivative even though the stock price was below the strike price. This is rare and can signal either unusual conviction or specific contractual reasons (expiring options, for instance). Worth investigating but not automatically bullish.

Code C — conversion of a derivative security. A convertible note, warrant, or similar instrument was converted into common stock. This is typically a mechanical event tied to the terms of the instrument, not a market timing decision.

The rule of thumb for X, O, and C: look at what happens next in Table I.If the insider exercises and then holds all the shares (no S code follows), that’s a meaningful signal. If they exercise and immediately sell, it’s compensation liquidation.

Edge-Case Codes Investors Often Misread: G, W, D, K, U

These codes appear less frequently but generate disproportionate confusion when they do.

Code G — bona fide gift.The insider gifted shares to another person or entity—a family member, a charity, or a donor-advised fund. This is estate planning, not a sell signal. The insider didn’t receive cash; they transferred ownership. Gifting shares to a foundation is common among executives with large equity positions.

Code W — acquisition or disposition by will or laws of descent. Shares were received through inheritance or distributed from an estate. No living person made a trading decision here.

Code D — disposition back to the issuer.The company repurchased shares directly from the insider at an agreed price—distinct from a Code S open-market sale. Code D often appears when an insider forfeits unvested shares upon departure, or when a company claws back shares under a compensation recovery policy.

Code K — equity swap or instrument with similar characteristics. This is a modifier, not a standalone code. When a transaction involves an equity swap or total-return swap, the filer appends K to the primary code—so you’ll see “S/K” or “P/K” in the transaction field. A P/K (purchase via equity swap) warrants investigation, because swaps can provide exposure to share price without direct ownership, potentially reducing short-swing profit rule exposure under Section 16(b).

Code U — disposition pursuant to a tender offer.The insider tendered shares in a change-of-control transaction. This is a corporate event, not a voluntary selling decision. You’ll see U codes when a company is being acquired and insiders are required to tender their shares alongside other shareholders.

Indirect Insider Trades: The Ownership Form Column

Every transaction row on a Form 4 also carries an ownership form column: D for direct (the insider owns the shares in their own name) or Ifor indirect (shares held through a trust, LLC, family account, retirement account, or investment fund). This column is easy to overlook—and that’s exactly why it matters.

A 2023 study published in the Journal of Financial and Quantitative Analysis by Goldie, Jiang, Koch, and Wintoki found that indirect insider trades—those made through family, trust, retirement, or foundation accounts—are more profitable than direct trades. After two years, a hedge portfolio of large purchases and sales in nonfamily trusts earned 10.48%, versus 7.22% for nonfamily retirement accounts. Both significantly outperformed direct trade strategies.

The authors also found that indirect trades account for 33.2% of insider purchases but only 16.0% of sales, suggesting insiders use indirect accounts more deliberately when buying. The most likely explanation: indirect trades attract less regulatory and media scrutiny than direct trades, so insiders can execute them with more conviction and less noise.

The practical implication: when you see a Code P with ownership form I (indirect), treat it as at least as significant as a direct purchase—and potentially more so.

A Decision Tree for Reading Any Form 4 Row

You don’t need to memorize all 20 codes. You need a fast filter. Here’s a three-question framework you can apply to any Form 4 row:

If a filing passes all three checks—Code P, any ownership form, no 10b5-1 plan—then look at magnitude. The most powerful insider signal is not the absolute dollar amount but the purchase as a percentage of the insider’s existing holdings. A CEO doubling his position is a stronger signal than a director adding a token 500 shares.