What is the difference between the smallest six digit number and the greatest four digit number

The smallest (whole number) 6 digit number is 1,000,000

The greatest (whole number 4 digit number is 9,999

The difference between them means to subtract the four digit number from the 6 digit number.

The difference is 990,001

I would not consider 000001 to be a 6 digit number (in case this is supposed to be a trick question.)

Working with whole numbers only ... the smallest 6-digit number is 100,000, the greatest 4-digit one is 9,999, and their difference is 90,001.