What Is a Cluster Buy? Multiple Insiders Buying (Form 4)

A cluster buy is two or more insiders at the same US company disclosing purchases of its stock on SEC Form 4 within a short window. The term has no legal definition; it is a way of reading Form 4 filings, so anyone using it has to say how they count. Flock's live page Which stocks have multiple insiders buying? counts a cluster as at least 2 distinct reporting owners (by SEC owner CIK) who disclosed purchase transactions (Form 4 transaction code P) in filings that landed within the same 30-day window. On 19 September 2026 that page listed 104 US companies in Flock's Form 4 coverage meeting the test in the window 21 August to 19 September 2026, out of 1,209 purchase rows filed in the window. Not investment advice.

What is a cluster buy made of?

Every element of the definition comes from the Form 4 itself.

The filer. Section 16 of the Securities Exchange Act requires officers, directors and holders of more than 10% of a class of equity to report changes in their beneficial ownership. Each reporting owner has an SEC CIK, which is how two filings by the same person are recognised as one insider and two filings by different people as two.

The transaction code. Table I of Form 4 carries a one-letter code for each transaction. Code P is an open-market or private purchase. Code S is a sale, A is a grant or award, M is the exercise of a derivative such as an option, and F is shares withheld to pay tax on a vesting award. Only code P is spending the insider's own money to acquire stock, so only code P counts as buying in a cluster.

The dates. A Form 4 carries a transaction date and a filing date. Form 4 must be filed before the end of the second business day following the transaction (SEC Form 4, General Instructions, read 19 September 2026), so the two are normally close, but a late filing can carry an older trade. Flock's page groups by filing date and says so.

The window. Thirty days is a convention, not a rule. A shorter window produces fewer, tighter clusters; a longer one groups purchases that may have nothing to do with each other.

How Flock counts a cluster, in its own words

The method is printed on the live page, and it is worth repeating because the count depends on it. From the page's FAQ, read 19 September 2026: a cluster is "a US company where at least 2 distinct reporting owners (by SEC owner CIK) disclosed purchase transactions in Form 4 filings (transaction code P, which covers open-market and private purchases) whose filings landed within the same 30-day window". Grants, option exercises and tax withholdings are excluded; superseded filings are excluded.

The largest cluster listed that day was Taiwan Semiconductor Manufacturing Co Ltd (TSM): 30 distinct insiders across 33 filing rows between 4 and 9 September 2026, with a combined disclosed value of $276.0K. That example makes two points at once. A cluster can be many insiders making small purchases, so the insider count and the dollar value are different facts. And the page recomputes hourly over a rolling window, so the list you see will differ from the one described here; the figures in this post are dated for that reason.

Reading a cluster buy without over-reading it

It is a filing-date window. Two purchases whose filings land 29 days apart count; two whose filings land 31 days apart do not. The boundary is a convention.

Count and value are separate. Thirty insiders buying $276.0K between them is a different fact from one insider buying $39.4M, which was the largest single purchase on Flock's biggest insider buys this week page on the same day (Cascade Investment LLC in Republic Services, filed 14 September 2026).

Coverage is scoped. Flock ingests Form 4 filings for the US issuers in its coverage scope, the securities mapped from tracked institutional 13F holdings, not every SEC registrant. A cluster at a company outside that scope will not appear, and every count on the page is a claim about Flock's coverage, stated as such.

A 10% owner is an insider too. Section 16 covers holders of more than 10% of a class, so a fund or holding company that crosses that line files Form 4 alongside officers and directors. Its purchases carry code P like anyone else's and are counted the same way.

Purchases are one side of a ledger. The same insiders may have sold under a Rule 10b5-1 plan earlier in the year, or may hold options and awards many times the size of the purchase. The Form 4 shows the transaction, not the reason.

Where cluster buys appear on Flock

• Which stocks have multiple insiders buying?: the live list, the 25 largest clusters with each insider named and each filing linked, recomputed hourly, free without an account.
• Biggest insider stock buys this week: the largest single code-P purchases in a rolling 7-day window.
• Each purchase has its own page under /insider-buys/, carrying the reporting owner, role, shares, price, trade date, filing date and a link to the Form 4 on SEC EDGAR.

For the background, read what a Form 4 filing is, how to track insider buying, Form 3 vs Form 4 vs Form 5 and what a Rule 10b5-1 plan is. For the quarterly, slower disclosure that shows what institutions held, see what a 13F filing is.

A cluster buy is a count of filed purchases by distinct insiders inside a stated window. Flock reports the count, the dates and the filings, and leaves the interpretation to you.