That's just called a PCIe x4 [1]. Each PCIe lane is an independent channel. The wider slot will simply have disconnected pins. You can actually do this with regular motherboard PCIe x4 slots by cutting the plastic at the end of the slot so you can insert a wider card and most cards work just fine.

[1]: It sounds like a nitpick but a PCIe x16 with x4 effective bandwidth can exist and is a different thing: if the actual PCIe interface is x16, but there is an upstream bottleneck (e.g. aggregate bandwidth from chipset to CPU is not enough to handle all peripherals at once at full rate.)