21991 ÷ 4 = 5497 R3
We're going to perform division on 21991 with 4 as the divisor.

Step 1:
4 goes into 2 0 times with a remainder of 2.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 2 to get 2.
Bring next digit (1) of the dividend behind the 2 and repeat the process: 21 / 4

Step 2:
4 goes into 21 5 times with a remainder of 1.
Write down 5 as next digit of of the result. 
Result so far: 5
Subtract 20 from 21 to get 1.
Bring next digit (9) of the dividend behind the 1 and repeat the process: 19 / 4

Step 3:
4 goes into 19 4 times with a remainder of 3.
Write down 4 as next digit of of the result. 
Result so far: 54
Subtract 16 from 19 to get 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 4

Step 4:
4 goes into 39 9 times with a remainder of 3.
Write down 9 as next digit of of the result. 
Result so far: 549
Subtract 36 from 39 to get 3.
Bring next digit (1) of the dividend behind the 3 and repeat the process: 31 / 4

Step 5:
4 goes into 31 7 times with a remainder of 3.
Write down 7 as next digit of of the result. 
Result so far: 5497
Subtract 28 from 31 to get 3.

The final result is 5497 with a remainder of 3.