DDR4 is still more expensive than DDR3, especially if you want high-speed and low-latency memory that outperforms DDR3. So by picking a Z170 based motherboard with DDR3 support you will be able to push your DDR4 upgrade a couple of years down the road, depending on how often you upgrade your system.
For those that might consider upgrading to DDR4 though and want a motherboard to support that, there are some options. The most notable one is the Hi-Fi H170Z3 from Biostar which has 2 DDR4 and 2 DDR3 slots. Given that DDR4 supports 16GB DIMMs, it’s possible to put up to 16GB of DDR3 in now and later upgrade to a max of 32GB of high-speed low-latency DDR4 ram. Hopefully, there will be more boards from other firms for dual DDR3/DDR4 support.Source: eTeknix