Insert a Zero after a specific number of character in a string

I have a string field called [PROJECT_NO]. It has 7 characters in the string. Example: 02WG028
We are changing our project number standard to eight characters, so I need to "insert" a zero after the 4th character from the left in the project number.
Attachment 24004
I have spent almost two whole days trying to figure this out on line with no luck. My head is going to explode!

Thanks in advance for any help.

P.S. this layer is an SDE layer that I am editing using a version.

Attached Thumbnails