Section 4. If the department determines that the topography of the area adjoining the highway does not permit adequate screening of such junkyards or that the screening of such junkyards is not economically feasible, it may acquire by purchase or otherwise or take by eminent domain under chapter seventy-nine such land or interests in land as it deems necessary to secure the relocation, removal or disposal of the junkyards. The department shall pay the costs of such relocation, removal or disposal as approved by said department. If the department determines it to be in the best interests of the commonwealth, it may acquire by purchase or otherwise or take by eminent domain under chapter seventy-nine such land or interests in land as may be necessary to provide adequate screening of such junkyards.