OK, let's do this. We've got 4221 and 4224 and we're adding them all together.

Step 1: We'll start by adding the digits 1 & 4 in column 1 and get 5.

Step 2: We'll start by adding the digits 2 & 2 in column 2 and get 4.

Step 3: We'll start by adding the digits 2 & 2 in column 3 and get 4.

Step 4: We'll start by adding the digits 4 & 4 in column 4 and get 8.

So, 4221 + 4224 = 8445.

