removing a specified number of characters at the end of files of various lengths

So I recently switched programs for file conversion, and the new more affordable program cannot number in batches but by files only. So i am left having to renumber the entire 3000-10,000 page batch. the format is as follows.

001_123456789^Doe,John Q0001
002_987654321^Dough,John Q0001

I need to be able to remove the last 4 characters and replace with incrementing numbers. I can add the numbers i need but as the file names are different lengths i find myself having to remove the 4 digits for each file. any suggestions on this?

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Reply to #1:
AS A DISCLAIMER, I AM NOT GREAT AT THE COMPUTER STUFF. SO THERE IS PROBABLY A SIMPLE ANSWER I JUST DON'T KNOW HOW TO APPLY.

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