You can't. All you can do is manually reset the counter to 0000. I rename all my image files anyway; I shoot with multiple bodies and have to in order for proof galleries to sort in chronological order. And having shot over 70000 frames, I have to rename or I'll have several files with each possible filename. Photoshop has a very useful batch rename tool. I use it to rename my files to YYYY-MM-DD-XXXX, where Y, M and D are year, month, and day, and X is a 4-digit counter that starts at 0001 each day.

The image numbers on my new 20D have suddenly jumped from below 1000 to over 9000.  The only reason that I can think of for this sudden change is that I used the memory card from my Canon G3 in the 20D (the G3 images are correctly numbered just over 9000).  I formatted the card from the G3 while it was in the D20.  Does this make sense?

So in the near future, my 20D image numbers will start over again.  But this would mean that my 20D images would be numbered (roughly) 0 to 1000 and 9000 to 9999.  How can I reset the numbering so that numbers 1000 to 9000 are used?

I had a strange experience with the first shots I took with my brand new 20D and brand new Sandisk Extreme 1GB card.

I didn't even format the card before using it, but when I downloaded the first RAW images, they were divided between 2 folders, 899 and 900. Folder 899 contained the bulk of the images (IMG 9906-9999) and folder 900 contained images starting again at 0001.

If I wasn't a completely trusting and naive person, I'd think that either the camera or card was not new  ??? .

Is there any other explanation?