Let's solve this addition problem. We have 4104 and 21, and we need to add them together.

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

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

Step 3: We'll start by adding the digits 1 & 0 in column 3 and get 1.

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

So, 4104 + 21 = 4125.

