It seems that samba_backup script is not purging two files of backup.

Samba4 has 
find $WHERE -name "samba4_*bz2" -mtime +$DAYS -exec rm  {} \;
and works for this files

But backup also has two more files, please add this:
find $WHERE -name "etc*bz2" -mtime +$DAYS -exec rm  {} \;
find $WHERE -name "sysvol*bz2" -mtime +$DAYS -exec rm  {} \;

Tested and works with .../source4/scripting/bin/samba_backup

Thanks