Okay, we are tasked with adding 426 and 4003. Let's begin!

Step 1: We'll start by adding the digits 6 & 3 in column 1 and get 9.

Step 2: We'll start by adding the digits 2 & 0 in column 2 and get 2.

Step 3: We'll start by adding the digits 4 & 0 in column 3 and get 4.

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

So, 426 + 4003 = 4429.

