I need a formula that will allow a set of numbers to go from this:

xxxxxxxxxxxxxxx

to this:

xxxxx-xxxx-xxxx-xx


Every formula I've come across will only let me add three hyphens to a set of numbers, where as, I need to add four. I really don't want to have to do this manually. The hyphens in the example are placed where I actually need them to be.

Can anyone help?!