What is the difference between the smallest 6-digit number and the greatest 4-digit number?

The smallest 6-digit number is 100000, while the greatest 4-digit number is 9999.

To find the difference between these two numbers, we perform the following calculation:

100000 - 9999 = 90001

So, the difference between the smallest 6-digit number and the greatest 4-digit number is 90001.