The operation is:
Because, the 2 from 4002 is less than the 3 in 2153 we have to borrow from the new number.
In this case before the 2 in 4002 we have two zeros than can not borrow to 2, so the 4 will borrow an unit to the next number (the zero in this case), and the zero will convert in a 10 and the 4 in a 3. Now we have a zero taht convert in a 10 and can borrow an unit to the zero next to it, so the 10 convert in a 9 and the zero in a 10.
Finally, now we have a 10 before the 2, so it can borrow an unit and it will convert in a 9 and the 2 in a 12. Now, 12 is greater than 3, so we can substrac.
In the picture you can see how the numbers borrow a unit between them and convert.