Section 4. Each register, with the approval of the superior court, may appoint an assistant register of deeds, removable at his pleasure, for whose official acts he shall be responsible; provided, however, that any person so appointed by the register for Suffolk county who has served continuously in any position in the registry of deeds in said county for a period of twenty or more years shall not be removed therefrom except for cause. The assistant shall give bond in the same manner as the register, and may be a woman. Any document or paper certified or attested by such assistant or by a second, third or fourth assistant register or by a chief clerk designated under section seven shall be admissible evidence as if certified or attested by the register.