Well that depends on a few things, mostly what you are planning on using the computer for now and in the future.
I will go through some pros and cons of each. Since both sets have the same frequency I will leave that variable out.
2x4GB = 8GB
This option is obviously nice if you plan on upgrading your ram in the future, it gives you 2 free slots so that you don't have to buy 4 new ram sticks later you can just buy 2 that match the ones you already have. Since I don't like making assumptions I will give you a couple warnings ... make sure you are running a 64bit OS and make sure your mobo can support 4GB sticks. On any new rig I would assume you should be fine but like I said its better not to assume.
4x2GB = 8GB
This setup is nice mainly for cost saving reasons. That is, ram goes on sale quite often so if you are buy 2 packages (two 2x2GB packages) you will double any of the savings you get from one. aka, you may see ram go on sale for $20 off/package, if you buy 2 that is $40 saved! However you will rarely see a single package go on sale for $40 off/package.
If you are not doing anything that will tax your ram usage very much (video, heavy photo editing) then you probably will not have to ever upgrade to 16GB so this set up could be perfectly fine.
A word on OCing your ram. It is generally harder to OC 4 ram modules than 2, simply because you have a higher chance of getting one module with a lower limit than the rest. However, it is also usually harder to OC larger ram modules simply because you have more ram chips that could limit the OC. So for OCing it is pretty much a toss up although if I had to pick I would go with 2x4GB for OC.