Why they revert to single channel at the higher speeds puzzles me.
Also, being a generic memory kit (rated at JEDEC specs - 1333 CL9 1.5V), you really should be expecting no overclocking ability.... if you want any overclocking ability and binning of any sort, you should've taken the step up to DDR3-1600. Try raising the vDIMM voltage, but I doubt you will get fully stable results with a vDIMM raise since most 4GB DIMM generic kits happened to be binned even lower than 2GB DIMM kits.
I blended at 1573, stock timings/voltage for a few hours on stock cpu speed. But now that im overclocking the CPU it is not passing blend anymore.
Generic RAM is often known for holding back CPU overclocks, especially on multiplier-locked processors.