I ran into this issue myself, so I figured I'd make a quick post on the off chance anyone else looks into it. The Dell XPS 8700's motherboard has an x4 slot, but it's not wired as such. This is contrary to the official specifications published by Dell (PDF) which, at the time of this post, read:
Expansion Bus
PCI Express x1:
- Connectors two
- Connector size 36‑pin
PCI Express x4:
- Connectors one
- Connector size 64‑pin
PCI Express x16:
- Connectors one
- Connector size 164‑pin
PCI Express mini‑card
- Connectors two
- Connector size 52‑pin
In any other PC spec where the x4 is wired as an x1 that I've seen, it's always specifically mentioned so as not to mislead the consumer.
Sadly, this means that if you'd like to upgrade your system to a shiny new nvme SSD via a PCIe adapter, you're out of luck. Unless you're not using a graphics card, of course. Having an available x4 slot is one reason I'd bought this system as a base. Is this something actionable for false advertising? I don't know.