How to format String with a pattern

I want to format my string in my select statement but I cant seem to find the syntax for this pattern. I have a column of card_num with 16 digits and i want to have a dash(-) after the first four digits. Original card_num: 1234567891234567 Desired output: 1234-567891234567 I think regex can do this but cant find elsewhere the right code for this pattern. Any idea how to do this? Thanks.