I need replace the contents of a column as follows

The column will have have empty rows rows with 4 characters and rows with
8 characters.

For the rows with 8 characters I need to insert a "/' in between.

e.g.

XXXXWWWW - XXXX/WWWW.

I know I can use ???????? in the search, but how do I get the replace to
only insert the "/" in between the characters?

The first four characters are not always the same.

Thank you for your help

JFercan.