Just buy now
ddr4 isn't going to be that amazing upon release and it certainly isn't going to be anywhere near the price point where people will be wanting to buy it.
Your build now will be no less "future proof" than a new one 6 months from now. If you wait until october/november, someone else will come along and say "wait for the XXX that comes out in april" . That's how technology is, seems to never stop
Futureproofing is really just a way for you to spend more money now for something that may or may not be useful in the future. Just my opinion.