Section 4B. The board of trustees, in electing a teacher in a state university who has served as such for the three previous consecutive school years, shall employ him to serve at its discretion, and notwithstanding any contrary provision of general or special laws, he shall not be dismissed from such employment except for just cause and for reasons specifically given him in writing by the said board. Before any such removal is effected, the said teacher, upon his request, shall be given a full hearing before said board, of which hearing he shall have at least thirty days written notice from said board, and he shall be allowed to answer charges preferred against him, either personally or by counsel.