mm 4 means Observed
The match came from a first-party observation without authentication, such as a first-party cookie or local storage ID.
When to send it
Send 4 when the ID was tied to the user through a first-party identifier the publisher or its partner set and later read back: a first-party cookie, an ID kept in local storage, a session ID or a partitioned cookie. Nobody logged in, but the observation is direct rather than inferred. Name the matcher in EID.matcher and set EID.mm to 4.
Buyers treat observed matches as reliable within one site or app and weaker across properties. The value is confused with 2 (Browser Cookie Sync), which relies on third-party cookies, and with 3 (Authenticated), which needs a login. If the first-party ID was later linked to other devices by a graph, the link itself is 5 (Inference).
Example
{
"user": {
"eids": {
"mm": 4
}
}
}Fields that carry it
Other values in id match methods
The id match methods list page explains the whole list and its common mistakes.