Folder of e-mails in maildir format had been corrupted. Typical file name is 1246281161.6777.m21JH:2,S . The " :2,S prevents " copying to another device. How can I simply remove the last four characters?

Usually if there are characters in a filename that are metacharacters - they mean something to the shell - you can surround the filename with double quotes or single quotes (tic) and copy the file or rename it. To remove the last 4 chars in the filename means you have to feed it to the mv command. Which the same as feeding it to a cp command

Try something like this to rename the files:

#!/bin/bash
ls ./maildir |
while read filename
do
   len=${#filename}
   len=$(( $len - 4 ))
   printf "mv '%s'  '%s'" "${filename}"  "${filename:0:$len}"
done > tmp.sh
chmod +x tmp.sh
tmp.sh

for i in *:*
do
mv $i `echo $i|cut -d":" -f1`
done

echo $i|cut -d":" -f1