Provide the ID of professors who are teaching high-level or harder undergraduate course.
ID of professors refers to taughtBy.p_id; high-level or harder undergraduate course refers to courseLevel = 'Level_400'
SELECT T2.p_id FROM course AS T1 INNER JOIN taughtBy AS T2 ON T1.course_id = T2.course_id WHERE T1.courseLevel = 'Level_400'