It runs in dual-channel with all 4 slots filled, just at 1333 MHz, at least my G53SX does this. ASUS basically built a multiplexer that splits the channels to two slots each. memtest86 reports it running in dual-channel mode. Only the desktops support more than dual-channel hardware, but multiple memory SODIMMs per channel is possible (and existing on ROG notebooks, as well as a few others).
From what I can tell, the IMC--for Sandybridge--cannot handle 4 slots of 1600 MHz, but it still runs it in dual-channel mode. Like @fuzon1337 said, slots 0 & 1 are one channel and 2 & 3 are the other. Adding more--matched!--RAM should not change the channel mode, just possibly the memory frequency and timings.
