Lusting after something isn't bad in and of itself; just make sure you don't act on it untill "a girl 4-8 years younger than you" is of legal age herself.
Really, if the point (to you) is that you are attracted to those who are 4-8 years younger, that isn't all bad, once you yourself are 30 orso, which brings your 'interesting age-range' into 22-26, which is perfectly acceptable.
Of course, if the issue is "underaged girls", there is a different point, and you might want to change psycologists...
You aren't a bad guy for this, awareness and admitting these things to yourself are the biggest steps, which you have already taken.
Try to find out which of the two situations i mentioned (4-8 years younger, which happens to be underage, for now VS becouse they are underage) is true for you, and react accordingly, since the 1st situation will fix itself in roughly 10 years.