Okay, we are tasked with adding 4321 and 1665. Let's begin!

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

Step 2: We'll start by adding the digits 2 & 6 in column 2 and get 8.

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

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

So, 4321 + 1665 = 5986.

