The user-IDs of members of the University are four digits.
Should a student be studying for longer time at this University, said student might have an eight-digit user-ID. After changing the study program or newly enrolling (not re-registration), the old user-ID gets decommissioned and a new one generated.
At re-registration the user-ID persists.
If you already have a four-digit user-ID, you will keep it forever.