Is there anyway with Tomcat 4.1.18 to view how many users are connected to my web application at any given time?
Also, is there a general rule of thumb for how much memory an orphaned session uses? I have my inactivity timeout set to 30 minutes and I am wondering if my users are just closing the IE window without properly logging out, leaving an orphaned session in memory for 30 minutes.
I would not worry about "orphan" sessions - Tomcat is allowed to serialize sessions to disk to manage memory. The minimum size session is tiny, memory use depends entirely on the references you store there - no generalization is possible.
The "manager" application for Tomcat will show you the number of active sessions - it is impossible to tell exactly the number of users "connected" but the number of active sessions will be the max. As I recall, the manager app was not part of the standard Tomcat 4 download for security reasons.
I upgraded from Tomcat 4 to 5 a number of years ago with no trouble.