💎 A session in GA3 is a collection of hits logged for a user over a specific period of time. A session in GA4 is a collection of events that are logged for a user during a specific period of time.

💎 The time between the first and last hits in a GA3 session are used to determine the session's length. While the time between the first and last events in a GA4 session is used to determine how long the session lasted.

💎 In contrast to GA3, user activity in GA4 is automatically identified. This could increase GA4's active user numbers.

💎 A GA4 session starts to expire whenever a website or app is put in the background.

💎 The timeout for a GA3 session cannot be less than one minute or more than four hours. In contrast, the timeout for a GA4 session cannot be less than 5 minutes or more than 7 hours, 55 minutes.

💎 If a person visits your website again from a different campaign source while a GA3 session is still ongoing, the session ends. However, if a person visits your website again from a different campaign source while their session is still open, GA4 does not end.

💎 GA3 uses the sessions statistic to tally sessions. GA4 counts sessions by recording the number of session start events.

💎 At midnight, a GA3 session automatically ends (11:59:59 pm). A GA4 session does not, however, end at midnight on its own (11:59:59 pm). The identical GA4 session, however, would now count twice - once for each day.

💎 Sessions are calculated and adjusted in various ways, the session count of GA3 and GA4 is unlikely to match.

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GA4 [UA→GA4] Google Analytics