How does Heap define a session?
A session is a period of activity from a single user in your app or website. It can include many pageviews or events. For web, a session ends after 30 minutes of inactivity from the user. For iOS, a session ends after your app has been closed or has entered the background.
Also, 4 server-side events are priced the same as a single session. Server-side events are counted differently because they aren't associated with any sessions.