Form 4 is the filing that makes US insider dealing public. Officers, directors and beneficial owners of more than ten percent of a company's stock must report changes in their ownership to the Securities and Exchange Commission, and those filings are published as they arrive.
The deadline is unusually tight — two business days from the transaction — which makes the US regime one of the most current disclosure systems anywhere. By the time you read a Form 4, the trade is typically days old rather than weeks.
The critical field is the transaction code. Code P is an open-market purchase: the insider bought at the prevailing price with their own money. Code A is an award or grant, code M an option exercise, code S a sale, code F shares withheld to cover tax. Those codes are the difference between a purchase and a payroll event, and reading a Form 4 without them is close to meaningless.
Ten-percent owners complicate the picture. A fund that crosses the ownership threshold files Form 4s like any insider, but its buying reflects a portfolio decision rather than management's view from inside the business — so a very large filing is sometimes an institution rebalancing rather than an executive backing their own company.
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Information only, not investment advice.