it's not a matter of deciding whether to go 4x4GB or 2x8GB, instead it is a matter of getting sticks from same brand and model, if you mix models, you could get instability issues. Some are solved by manually setting the latencies of the slowest sticks in the BIOS, but not 100% accurate.
Whether to go 4x4GB or 2x8GB of the same brand and models, there is no noticeable difference in performance, you could get whatever is cheaper.
So, if money is not a problem, I would get 2x8GB and sell the other 2x4GB elsewhere.