rule 1 /^9\(.*\)/ /4\1/
\1 means the first group which is (.*) in this case.
/ ^9 \(.*\) / as you understood the first part correctly, the second part is grouping all the coming numbers after 9 and make it a group.
/ 4 \1 / then in the translation as you mwntioned replace 9 with 4 and then follow the 4 with the Group 1 which you previously determined in the first part of your rule.
i hope that this will help you.