Round trunnions b of link 3 fit into holes in prongs a of link 1. Hole d of link 3 fits over round guide c of link 4. Square trunnions e at the ends of link 4 slide along guide slots f of link 2. With respect to each other, links 1 and 2 accomplish two turning motions about perpendicular axes x-x and y-y, and two translational motions along perpendicular axes x-x and z-z.
$90$EM,MJ$