Section 4. The commissioner, or such other of his assistants as he may designate, may summon a licensee, or any of his agents or employees, and such other witnesses as he deems necessary, and examine them relative to their transactions, may require the production of books and papers and, for such purposes may administer oaths. Whoever, without justifiable cause, fails or refuses to appear and testify or to produce books and papers when so required, or obstructs the commissioner or his representatives in the performance of their duties, shall be punished by a fine of not more than five hundred dollars or by imprisonment for not more than six months, or both.