RAM can be kinda finicky. Mixing and Matching RAM, even of the same model, does not always work. This is why RAM is sold in kits as the sticks are tested with each other to ensure compatibility.
Your best option is to look for another stick of the Ballistix Sport. But there is a chance that it wont work. If you go with the XMS3, there is a bigger chance that it wont work.
If you want to guarantee that it will work, buy a 8gb kit and sell your 4gb stick.