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To understand this, you need to understand how GA4 calculates its audience size. The equation (1 < 3 < 4 = 2) signifies that the audience size varies based on the scope conditions in GA4 and it doesn't always correlate with the number of sessions that meet your criteria in a linear way.
The scope can be set at "User", "Session" or "Event". GA4 measures audience condition scope differently. For "User" scope, if any user session or event meets the criteria within 30 days, the user is included in the audience size. For "Session" scope, if any session meets the criteria in 30 days, it's counted in the audience size even if the same user had different sessions. For "Event" scope, GA4 counts each individual event that meets the criteria within 30 days.
If we take John's example, according to "User" scope (condition 1), John is counted as one user since he met the criteria within 30 days. In the "Session" scope (condition 3), John contributes three since he had three sessions that met the criteria. Using "Event" scope (condition 4), John contributes two, given if he only triggered two events that met the criteria, regardless of his sessions. Condition 2 wasn’t mentioned clearly in the question.
That's why the audience size could appear as you've witnessed (1 < 3 < 4 = 2). The number of sessions that meet the criteria doesn't always directly correspond with your audience size, as it also depends on the condition scope.
