Let's divide 639666 by 4
We're looking to find how many times 4 goes into 639666.

Advancing to step 1:
When dividing 6 by 4, we get 1 with a remainder of 2.
The next digit of our result is 1. 
Result so far: 1.0
Subtract 4 from 6 to get 2.
Grab the next digit (3) from the dividend, add it to 2, then carry on: 23 / 4

Moving on to step 2:
4 can be fit into 23 5 times, resulting in a remainder of 3.
Put 5 as the next digit of the answer. 
Result so far: 15.0
Deduct 20 from 23 and we're left with 3.
Include the next digit (9) from the dividend after 3, then repeat: 39 / 4

Step 3:
4 can be fit into 39 9 times, resulting in a remainder of 3.
The next digit of our result is 9. 
Result so far: 159.0
Subtracting 36 from 39 leaves us with 3.
Fetch the next digit (6) from the dividend, attach it to 3 and continue: 36 / 4

On to step 4:
4 can be fit into 36 9 times, resulting in a remainder of 0.
Record the quotient 9 as the next digit in the result. 
Result so far: 1599.0
If we take 36 away from 36, we end up with 0.
Append the next digit (6) from the dividend to 0 and continue with: 6 / 4

On to step 5:
6 divided by 4 is 1 with a remainder of 2.
The next digit of our result is 1. 
Result so far: 15991.0
Subtracting 4 from 6 leaves us with 2.
Grab the next digit (6) from the dividend, add it to 2, then carry on: 26 / 4

Step 6:
If we divide 26 by 4, we get 6 and a remainder of 2.
Record the quotient 6 as the next digit in the result. 
Result so far: 159916.0
Subtract 24 from 26 to get 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

On to step 7:
When dividing 20 by 4, we get 5 with a remainder of 0.
The next digit of our result is 5. 
Result so far: 159916.5
Deduct 20 from 20 and we're left with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 8:
0 divided by 4 is 0 with a remainder of 0.
Write down 0 as next digit of the result. 
Result so far: 159916.5
Deduct 0 from 0 and we're left with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

On to step 9:
4 goes into 0 0 times with a remainder of 0.
The number 0 becomes the next digit in our result. 
Result so far: 159916.5
Subtracting 0 from 0 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

The final outcome is 159916.5, with a leftover of 0.0.