I may be mistaken, but the big advantage of DDR4 is higher clock-speed at RAM-sizes > 512GB.
IIRC, you can still get 1800MHz on a full TB of RAM, wheres with DDR, you were limited to 1333 or less (on a typical 2 CPU server).
As you might have guessed, unless there's a significant power-saving effect with DDR4 vs. DDR3 the benefit to something like a MacBook is probably in the region of nil.
But I'm also waiting for the 2nd generation rMB.
This Asus notebook already comes with Skylake:
http://store.asus.com/us/item/201510AM150007436/A17602
So, CPUs should be available. The question is: when will Apple release their updated MacBooks?
It's actually in store at various local dealers. The QHD+ version shows 2-3 weeks of delivery-time, though (and has about the entry-level price of the rMB).
Would Apple drag this until over the holidays?