Why would you buy a computer (especially laptop) to last four years??? Computer technology moves very fast, and four years is a long, long time in the world of technology. I'd strongly suggest you buy a computer expecting to use it for no more than two years. After two years you'll likely be ready for a new one anyway.
Personally, when it comes to laptops I'd say buy the cheapest decent one that does what you need and replace it as often as you need and can afford. I look at mobile technology as disposable, not only because it's quickly outdated, but because it also tends to get abused, lost and stolen.