"As a result, the X399 Aorus Xtreme motherboard sports four PCIe 3.0 x16 slots to support up to four graphics cards in an SLI or CrossFire configuration."
Please cite a single article which shows the utility of SLI/CF using more than 2 GPUs in any modern title. Devs aren't bothering so much with such support these days.
And hardly perfect, took seconds to spot that with a longish GPU in the bottom slot, the debug LED display would be blocked (had that issue with ASUS boards way back). The display should be elsewhere, typically top right, or better still fit two of them which display the same info.