var a = 3;
var b = 3;
var c = 3;
var d = 3;

if(a == b && c == d || a == c && b == d || a == d && b == c) {
  console.log("Mozna zbudowac prostokat");
  if(a == b && b == c && c == d) {
    console.log("Mozna zbudowac kwadrat");
  }
}