How to delete the first and last two characters of a text file using sed?

I have a text file like this

abcdefg
abcdefg
abcdefg
abcdefg

How to delete the first and last two characters from each line with sed? I would like to get the output like this

bcde
bcde
bcde
bcde


sed  's/^.\(.*\)..$/\1/'  file