This would be a bug in the JVM bundled with JBuilder 4. See this entry on Sun's Bug Parade for details and status:

http://developer.java.sun.com/developer/bugParade/bugs/4395735.html

Known workarounds:

1. Disable HotSpot. This may cause some slight reduction in JBuilder's performance but should eliminate the memory leak.
   1. Edit jbuilder4/bin/JBuilder.config and add the line:

      vmparam=-classic

   2. Restart JBuilder.
2. Replace Sun's 1.3.0 with IBM's 1.3.0 or better. (This is not a supported configuration.)
   1. Install IBM's 1.3.0 (perferably in a different location than the existing JDK.)
   2. Edit jbuilder4/bin/jdk.config and replace all references to Sun's JDK with the corresponding locations in IBM's JDK install tree.
   3. Restart JBuilder.
3. If your own applications experience out-of-memory errors while running, open "Project Properties | Run | VMParameters" and add "-classic" to the list of parameters applied to the application's launch command. This should prevent the problem when running within JBuilder.