var vs. let inside a for loop

Question

The following piece of code will produce 1, 2, 3, 4

const array = [1, 2, 3, 4];

for (let i = 0; i < array.length; i++) {
  setTimeout(() => {
    console.log(array[i]);
  }, 1000);
}

Whereas using var i = 0 in the for loop will produce undefined, undefined, undefined, undefined

const array = [1, 2, 3, 4];

for (var i = 0; i < array.length; i++) {
  setTimeout(() => {
    console.log(array[i]);
  }, 1000);
}

I understand var and let have different scopes but can somebody explain why in the var example, i evaluates to 4 in each iteration?