Audit table insert restriction
You probably want to use something like,

CREATE OR REPLACE TRIGGER usr_t_audit_user_alnd AFTER LOGON ON DATABASE
BEGIN
  -- insert audit record only if user is one of specified, else ignore
  if ora_login_user in ('USER_A','USER_C','USER_D') then
    INSERT INTO usr_t_audit_user (username, session_id, logon_day, logon_time)
    VALUES(UPPER(USER), SYS_CONTEXT('USERENV','SID'), SYSDATE, TO_CHAR(SYSDATE, 'HH24:MI:SS'));
  end if;
END;