You don't need to match it, you will need to take your existing two modules out anyway and put in 2x2GB (there are only two slots).
That aside, you can get any memory, as long as its a 2 GB DDR2 667 SODIMM module, I got the cheapest available (Adata) and I have been very satisfied (it comes with lifetime warranty so i don't really worry). Since all newer Macs are based on generic PC hardware, memory compatibility is no longer a problem, although some producers still try to trick you into buying special "mac memory" that is exactly the same as any other memory (except they perhaps tried if it works).
Also, don't worry about memory speed. There is no point in buying faster, 800 MHz memory, because it will still work at 667.