How to generate random looking unique identifiers that match a given pattern

I have a users table and I need to create a unique, random looking alphanumeric "id" for each user (they already have autoincrementing ids as usually). This identifier must:

• Be unique
• Be random looking
• Match the pattern AAAA-1234 (4 letters, 4 numbers)

Is there a better way than to keep randomly generating strings until I find one that is not in the database yet?