When you dismiss an employee, he or she has the right to know the exact reasons. To avoid future discussion or even a court case, it is best to specify the precise reasons for the dismissal on the C4.

Obligation to justify the dismissal – A clear C4 may be sufficient
Since 1st April 2014, a dismissed employee has the right to know the concrete reasons which led to his/her dismissal.

The employer is not required to justify each dismissal automatically – you must do so only if the employee asks you to. If you receive such a request, you must provide the exact reasons by registered post within 2 months from the date that the employee asked you (also by registered post). However if you have already provided the specific reasons in the C4 then no further justification is required.

Source: http://www.hralert.be/fr/news/%C2%AB-motivez-directement-le-licenciement-sur-le-c4-%C2%BB (HR Alert)