In my somewhat limited experience I would be surprised if either of these items needed replacing before 50k miles. There will be other variables as well, but all things being equal they should be reasonably durable. If either failed or started to give problems you would know about it because the car would either not be running as smoothly as it should or you will get an emissions light come on. Car should still be driveable and simply change then. I think way too much is made of these items being changed at 4 years when more often than not their actual design life is way longer. The only consideration would be if you were thinking about selling the car and they had not been changed, a potential buyer may take this in to account as an up coming cost, but no more so than a new clutch or brake pads and discs. If planning on keeping it then I wouldn't worry, change if there is an issue or when the book says it is mandatory, as opposed to recommended.