Okay, we are tasked with adding 4032 and 21. Let's begin!

Step 1: We'll start by adding the digits 2 & 1 in column 1 and get 3.

Step 2: We'll start by adding the digits 3 & 2 in column 2 and get 5.

Step 3: We'll start by adding the digits 0 & 0 in column 3 and get 0.

Step 4: We'll start by adding the digits 4 & 0 in column 4 and get 4.

So, 4032 + 21 = 4053.

