Section 4. The selectmen of contiguous towns shall, at the joint and equal expense of such towns, erect permanent stone monuments at every angle of their respective boundary lines and wherever a highway crosses such lines, unless such monument, two feet high from the ground, already exists or unless such lines are bounded by the sea or by a permanent stream. The monuments shall be well set in the ground, at least four feet high from its surface, and shall have the initial letters of the respective names of such towns legibly cut thereon.