Best to buy 2 x 4GB before DDR3 gets too out of date and the prices hit DDR2 levels.
In fact, you might want to try a single 8GB module; if that works, they you know the board will probably take 16GB, even though the spec says otherwise.
Dont worry if the RAM has a higher speed than the board can handle, it will clock down to board speed on its own.
Often, these boards are give a spec based on what is available at the time, I have had DDR3 boards claiming only 4GB per module that were happy with 8GB per module.
The same happens with phones; my first Xiaomi phone spec (from 4 years ago), said "microSD upto 64GB", yet it happily runs a 128GB card, and can probably handle much larger.