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A Form 4 is the disclosure an insider — an officer, director, or holder of more than 10% of the stock — must file with the SEC within two business days of buying or selling their own company's shares. It names the person, the date, the number of shares, the price, and whether the trade ran under a pre-arranged 10b5-1 plan. Canada's equivalent is a SEDI insider report; the UK calls it a PDMR notification.

Not front to back. The annual report's useful parts are Item 1A (risk factors, where new language year-over-year is the signal), Item 7 (management's discussion), Item 8 (the financial statements), Item 9A (internal controls), and the notes — which is where the things nobody wants found tend to live. Reading last year's alongside this year's and looking for what changed is worth more than reading either one alone.

It is the auditor stating substantial doubt that the company can survive the next twelve months. It appears in the audit opinion and the notes. It is not a prediction of bankruptcy, and plenty of companies carry one and recover — but it is the auditor, who has seen the books, saying out loud that they are worried.

A flaw in the company's own financial reporting controls serious enough that a material misstatement might not be caught. Disclosed in Item 9A. It does not mean the numbers are wrong; it means the process that is supposed to catch wrong numbers is not reliable. Restatements often follow one, which is why it is worth tracking.

Sometimes fee disputes or routine rotation. Sometimes because they cannot get comfortable with what management is telling them. The company must report the change on an 8-K under Item 4.01, and must say whether there were disagreements. The interesting part is usually the timing and what the outgoing auditor's own letter says — that letter is filed as an exhibit, and almost nobody reads it.

Both disclose a stake above 5%. A 13D means the holder intends to influence the company — activists file these, and Item 4 states what they want. A 13G is the passive version, for investors who are just holding. An investor switching from 13G to 13D is announcing a fight.

Search the company on SEC EDGAR and filter to Form 4. What matters is not any single sale but the pattern: what share of their holdings went, whether it ran under a 10b5-1 plan, whether several insiders sold in the same window, and what the company disclosed shortly afterwards.

Members of Congress must disclose trades over $1,000 in a Periodic Transaction Report, filed with the Senate's Electronic Financial Disclosure system or the House Clerk. The law allows up to 45 days to report, so the disclosure is always behind the trade — and the lateness is sometimes the story on its own.