Ok.. first off, you'll be able to run dual channel with either setup, so that's not an issue. (Dual channel just needs an even number of sticks).
Both options have their advantages - going 2x2 gives you room to update the ram later (assuming you are running a 64bit OS), but 4x1 will be cheaper and correspondingly for the same amount of money you'll be able to get faster ram.
The cheapest option would be to go with 4x1, then when you run out of ram (!) just sell two of them and get 2x2 which will then be cheap, giving you 6gb and staving off an upgrade for a while. By the time 6gb isn't enough you will be changing the rest of your system anyway.
So, that'd be my first choice if I was buying today. If 2x2 didn't cost too much more then I'd consider it just because it would use less power than 4x1, but the difference isn't huge, whereas the premium for 2gb sticks wasn't insignificant last time I checked.
Infact, do you actually need 4gb ram now? Cheapest still would be buy 2x1 and then plug in 2x2 later when the price comes down.