Honestly, people around here will rip someone apart for saying what I'm about to say. I've never had a problem with mixing manufacturers, as long as the RAM is of the same voltage, speed, and timings. <DISCLAIMER> I am not recommending that you do what I'm about to describe. </DISCLAIMER> Right now, I'm running the following in my RAM slots: 2 sticks of 4GB G.Skill Sniper DDR3-1600 CL8, One 2GB stick of Corsair Value RAM DDR3-1333 CL9, and one 2GB stick of Kingston HyperX DDR3-1600 CL9. It's rock solid on an MSI 790FX-GD70 motherboard. I don't know why or how, but it is. Not sure that it should be, but I'm not complaining. I don't really care as long as it works.
That aside, though, the truth is still that kits are sold generally for a reason - they're guaranteed to work together. Buying an entire 4-stick matched kit of RAM now, according to the manufacturer (!!), is "the only way to guarantee compatibility". From what I've observed, that's their way of getting you to spend more money than anything else.
My recommendation would be to get something as close as possible to your original kit. If it costs you $5 more, then it's worth it. When you send an OEM PC off to the repair shop to get a memory upgrade, they sure as heck don't throw away your old pair of sticks and order in a matched 4-DIMM kit. They find something that works and send it back to you.
So once again, get as close as you possibly can to your original kit, and you should be fine. Just my opinion, though... I'll let everyone else tell you what it's worth. Maybe I'm wrong. I just can't stand listening to people saying that "your only option is to get rid of what you already have and spend three times as much to get it back in a matched kit".