"Last Page" should only appear if the last contribution created a new page. It should show the title of the page that was created, the author who started the page, and the date it was first posted.
If any other edits to a page were made, it would be listed as "Last Edit".
If you are not seeing the title in the "Last" info, then it's possible the last update was made before some info started being tracked.
Also, edits to child pages within an area were also not considered as a "Last update" for the area until a recent version. Instead, only new pages, comments on the area itself, and edits to the area landing page itself were listed. I'm not sure what version you were using before you upgraded to 4.0.0, so I can't be sure if this was the case here or if the updates aren't propagating to parent nodes as intended.
Last-info that was generated on older versions is not automatically rebuilt by upgrades. The best way to make sure last-info is up-to-date is to make a newer update after the upgrade and see if the update field changes.