Is a whole number with 4 digits always greater than or less than a whole number with 3 digits explain

A whole number with 4 digits is always greater than a whole number with 3 digits.
This requires that both numbers be positive.

Explanation.
In the decimal system, we count 0,1,2, ..., 9 (1st digit).
When we reach 10, we shift left and count in tens from 10,20,30, ..., 90 (2nd digit).
When we reach 100, we shift left and count in hundreds from 100,200,300, ..., 900 (3rd digit).
Then we count in thousands as 1000,2000, ...  ( 4th digit).

Because the 4th digit counts in thousands and the 3rd digit counts in hundreds, a whole number in the 4th digit will be greater than a whole number in the 3rd digit.