It's because when you reach 9999 frame number, your camera won't fire anymore. You have to reset numbering (or create a new folder).
From my Canon user manual:
• Even if the memory card still has space, shooting is not possible beyond file number 9999. If this happens, follow the instructions on the LCD monitor and hold down the <SELECT> button and turn the <rear> dial to select [OK]. A new folder will be created and you can continue shooting.
To prevent any interruption in shooting, set the file numbering method to [M-Reset] beforehand.
• If the memory card in use has a DCIM folder, the file numbering might continue from the last image saved in the folder.
This is a feature and not a bug!
:cool: :cool: :cool:
Francois