Add _ based on a calculated colums

[stovaaa]

So I am creating new usernames for users in Excel. My format will look be AL11smitJ. The AL is the state the user resides in, the 11 is the Office location number, then I take the first four characters of the last name, and the first character of the first name. Here is the function I use:



=LOWER(CONCATENATE(J75,RIGHT(E75,2),LEFT(C75,4),LE FT(A75,1)))

My problem is that I need to lengthen last names that are less than four characters with underscores. I have a column that calculates the number of underscores needed to make the last name four characters long, but have no idea were to go from there.