map = {};
for (y = 2; y <= Bottom; y++) {
  key = [3,y];
  map[key] = [4,y];
}

for (y = 2; y <= Bottom; y++) {
  key = [1,y];
  if (key in map) {
    [3,y] = key;
    [4,y] = map[key];
  } else {
    [3,y] = "";
    [4,y] = "";
  }
}