Sonnet M.2 4x4 PCIe Card for SSDs - Sonnet - SONNETTECH
So, you can insert a PCIe 4.0 GPU into a PCIe 3.0 slot, but you'll be bottlenecked by the bandwidth limitations of Gen 3.
Can I Use a PCIe 3.0 Card in a 2.0 Slot? - PC Guide 101
The card takes a PCIe 4.0 x16 slot (can be run at PCIe 3.0 mode, for what it’s worth), and branches out into four PCIe 4.0 M.2 slots.
The M.2 slot: What is it and how can I use it?
However, inserting a PCIe 4.0 SSD into a PCIe 3.0 M.2 PCIe slot does not bring any advantages. On the other hand, a PCIe 3.0 SSD works fine in a PCIe 4.0 M.2 slot.