cube([20,20,10]);
$fn=1000;
translate([5,5,10]) cylinder(r1=3,r2=3,h=2);
translate([15,15,10]) cylinder(r1=3,r2=3,h=2);
translate([15,5,10]) cylinder(r1=3,r2=3,h=2);
translate([5,15,10]) cylinder(r1=3,r2=3,h=2);