I think that DDR4 would take effect when the memory is stupidly huge. Something like 512 gigs of RAM or more.
ddr3 capped at 2133 (the ram i have) and ddr4 started at 2133. ddr4 has a slower latency than ddr3, so theres a bit of a trade off when it comes to ram. without getting too technical lest just say you'll never see the difference between the two, but there is "for 2133", side by side. ddr4 jumps up to 3400, this is great news for onboard/apu gamers, because the system memory is shared with the gpu side of the cpu. faster = better for gaming. this same thing holds true for all things with the pc, the less of a bottleneck, the faster things can run. You dont really see much of a difference unless you fill in all of your ram slots. think of ram like a highway, you can pack 8 cars onto a single lane all going 65mph, that's great, we're all getting to our destination quickly. but once you add in another ram chip, another 8 cars can ride right beside them doing 65mph. What this translates to in a pc, is the cpu clock speed per core can be more effective when you've filled all your slots because it can use all the lanes at a single time to saturate the ram. a single stick will be the cpu and gpu bottleneck. Now if you plan on getting 16gb of ram, i wouldnt suggest doing 4x 4gb unless you never want to upgrade further. In practice it will be quicker, but limit you if you want 32gb down the road or you'll waste money throwing that ram out and getting 8gb sticks. 2x 4gb sticks will outperform 1x 8gb stick. Back on topic, You'll have performance improvements with ddr4, provided you're using faster ram than 2133, saturate your lanes with it, and keep your chipsets and drivers up to date*. (sometimes updates can actually slow things down to fix stability issues, so i added an asterisk.) will you visually see these differences, maybe, depends on your system, game, etc.