When you get the extra RAM, if you actually use the extra RAM, it will (through interleaving) revert to single channel which can significantly detriment your performance. You may also have compatibility problems mixing kits and kit sizes.
However, you can get 2x2GB kits often for the same price. Here's a LINK
to a particularly cheap DDR3-1600 CL9 kit. Or you will find the exact same one you have (best for compatibility) on sale for $33 shipped. Either way, it's the same price (or even lower) than a single 4GB DDR3-1600 DIMM.
Of course same rated speeds/timings (at the very least, voltage may or may not matter) are important, if you don't know the ICs used.
You may also want to go 2x4GB for the sake of expandability and IMC/memory controller stability.Edited by xd_1771 - 7/28/11 at 1:51am