The 4th rotary axis for 3040 and 6040 model:

Rotary worm gear with 0.2 deg return difference, with tail stock, it is suitable engrave metal

and wood

Rotary belt type  with tail stock, it is more suitable engrave plastic, wax mould etc Above all price include machine, electrical control box, test bits, clamps, wrench, carbon brush, parallel cable etc, it means you only need install the software can run it