No interest basically works like this. You will have your total due, say $900 if you buy the laptop. You will get a monthly bill b/c its just like a credit card, the minimum due will be 4%. You can pay JUST the minimum every month without getting hit with finance charges. BUT, if you don't pay off the entire purchase before the promotional date ends, you will get the ENTIRE amount of interest billed to you.
One other note, if you use the no finance option...do NOT use the card for anything else. When you make payments, it will go towards the promotional item ONLY and you will be charged finance charges for the new items that you bought.