A mixed number is a combination of a whole number and a fraction. The fraction 9/2 is equal to 4.5. So you would have to find a mixed number in between 4.5 and 5. You could do this by turning 5 into a mixed number, which would be 10/2.
So now you have to find a mixed number in between 9/2 and 10/2. The best way to do this is multiply both numbers so that you can get a wider range of numbers. You would not be changing the end result, you would only be changing the size of the fraction.
I would multiply both mixed numbers by 2, which would get you 18/4 and 20/4. That would mean that a number in between these two mixed numbers is 19/4. (which would simplify to 4.75)