This removes the first 3 characters (by output the 4th character onward) of input.txt, reverses each line, removes the first 3 characters of the reversed line (which are the last 4 characters of the non-reversed line), reverses each line back to normal, then outputs to a output.txt
Edit: if all your lines are 16 characters long then you can just do cut -b 4-12 input.txt > output.txt