The last four digits are being used on the Pre-day1 Temporary ID
SAP SuccessFactors Onboarding 1.0 - all versions
When the SSN is being sent from RCM module, the last four digits will be used to be part of the Temporary UserID.
This is an expected behavior from the system.
KBA , LOD-SF-OBD-INT , Integrations with Onboarding , Problem