In version 4 EA1, the Source menu item has been moved from the Edit sub-menu to the main menu_bar. However, it only is available when editing a cataloged PLSQL object (procedure, package, et cetera). I would suggest that it also be available when using the WorkSheet, as the functions in its sub menu (toggling comments, et altera) are useful when editing SQL or anonymous blocks in the Worksheet.
Agreed, that's a bug. Noted and logged as such.