Building a system will perpetually be a state of "wait for the next thing or go with what I have now?"
So far as I know, DDR5 is estimated to be coming in 2020. I've not personally seen any hard data on that number.
With that said, I think you're best off just building a system now. DDR iterations go through a cycle where they start off being fairly expensive/gb, early adopter tax, as it were. Then as they become "mainstream" and see wider adoption, they drop down in price, and when the next iteration comes along, and back inventory dries up, they become a niche item and become expensive again. DDR4 is currently in the "mainstream" phase, and now in particular is the best it's been in quite a while.