How to read a Form 4, and which insider trades are worth anything
Insiders file within two business days. Most of what they file is noise — scheduled sales and option exercises. Here is how to tell the difference.
A Form 4 is the filing a company insider must submit when their holding changes. The deadline is two business days after the transaction, which makes it one of the few genuinely fast disclosures in US markets — the information reaches you within days, not quarters.
Most of it still tells you nothing, and knowing which parts do is the whole skill.
The field that matters most is the transaction code
Every line carries a code. Only a few are worth your attention:
| Code | What happened | Signal value |
|---|---|---|
| P | Open-market purchase | High. They chose to buy, with their own money, at the market price. |
| S | Open-market sale | Low. Could be a house, a divorce, a tax bill. |
| A | Grant or award | None. The company gave it to them. |
| M | Option exercise | None on its own — but see below. |
| F | Shares withheld for tax | None. Mechanical. |
| G | Gift | None. |
The asymmetry is old and well documented: there are many innocent reasons to sell and essentially one reason to buy.
The pattern that is almost always noise
An M followed immediately by an S on the same or the next day is an option
exercise-and-sell. It shows up in a naive feed as a large insider sale and
means close to nothing — the insider converted compensation into cash, which
they were always going to do.
A feed that does not separate these will show you a stream of alarming “insider selling” that is mostly payroll.
The pattern that is worth a second look
A cluster of P transactions — several different insiders buying in the open
market within a short window, especially non-executive directors — is the
configuration with the most evidence behind it. One CFO buying is a data point.
Four officers buying in the same fortnight is a different kind of data point.
Size matters, but relative size matters more: $200,000 from someone whose holding was $250,000 says more than $2 million from someone holding $80 million.
Where it goes wrong in practice
Reading the dollar value without checking the price. Form 4 reports the price per share for the transaction. A “purchase” at a price far below market is usually an option exercise miscoded in someone’s pipeline.
Treating a 10b5-1 sale as a decision. Plans are set up months in advance precisely so that executives can sell without signalling. A sale under a plan is a calendar entry, not an opinion.
Aggregating without deduplicating amendments. Form 4/A amendments restate earlier filings. Sum them naively and you will double-count. We hold 5.2 million transactions across 2.1 million filings, and the parsing is the part that takes the work — a single mis-parsed filing once showed a $7.98 billion purchase by one insider, which is the kind of number that should stop a pipeline rather than appear on a screen.
The useful way to use them
Insider activity is a conditioning variable, not a trigger. It changes what you make of everything else on the page.
Open-market buying while institutional holders are reducing is a conflict, and conflicts are where the interesting questions are. Buying into a falling price after an 8-K is a different story from buying into strength. Neither is tradeable on its own; both change the reading of the rest.
That is also how our assistant is asked to use it: the insider block is one of eleven blocks in the evidence pack, and it is required to say when that block is empty rather than let you assume nothing happened.
Form 4 transactions sit on the same instrument page as the price chart, the filings and the institutional positions. Free to open.