Do the 5th gen H&R sport springs fit a 4th Gen?
Why do they have different part numbers, what is different about them?

 

I am not sure about this, but I heard before 5th gen springs came out, people would run 4th gen springs because the suspension is pretty much the same, but since the 5th gen is heavier, they would be getting lower than expected drops. With this in mind, a 4gen on 5gen springs would most likely ride higher than expected. As long as it doesn't introduce any other problems, you might be able to get away with it with the right springs.