Gabriel's Problem and Counting Factors might be good problems to try before looking at this one.
If you know the prime factors of a number, how can you work out which two-digit numbers are factors?
It might help to draw a factor tree.
If you'd like some help with the extension, click below
What is the largest that the four-digit number can be?
What is the smallest that the two-digit number can be?
What does this tell you about the three-digit number?
Now it might be helpful to think about the units digits.