If properly understood, the 82% increase is quite meaningful.
The bottom line is, whereas about 1 in 50 iPhone 3s experienced cracked screens in the first few months, 1 in 25 iPhone 4s are having this problem.
It was not “4 phones in 20,000” that broke but about 4% or about 800 or the 20,000.
Also, the fact that there is a reasonable explanation for the increase does not make it meaningless. If an automaker stopped putting locks on the doors and there was a 10,000% increase in thefts, that lack of locks would explain the rise, but would not mean it was not a problem.
In the same way, the front and back glass on the iPhone 4 should probably be considered a minor to moderate design flaw. Apple generally makes quality products, but I do think its focus on style of substance is weakness.