How so? They're marketing and promoting that the 4th gen fixes the problem. From a PR perspective its better to stand behind your product then automatically put it into the repair program - at least that's how I see it.
I see it as though they are standing by their product, the 4th gen keyboard, by covering it for 4 years out of the gate. They believe in it, believe that it won’t fail for as many customers and therefore they are putting their money where their mouth is.