You really need to answer Carbon's questions to be sure that you get the performance that you need on the tasks that you want to do. Post back and you'll get many opinions of recommended specs to accomplish your computing goals. One spec caveat, however, since you'll most likely end up with a Vista system - DO NOT get less than 2 GB of RAM; you won't be happy with the performance.
As to your arbitrary 4 year lifespan - ????
Where to buy: a) newegg.com. b) dell.com, c) hp.com, d) COSTCO or, costco.com. My choice would be to purchase from COSTCO, store or website, because they have competitive pricing and add a year to the warranty.
Manufacturer: HP, Dell, Acer, in that order.
You can get a quite nice machine today for your $1000. Look around and see what appeals to you.
Best of luck!