Modern connections using PCIe 4.0 lanes can work with older PCIe 3.0 devices.
Modern connections using PCIe 4.0 lanes can work with older PCIe 3.0 devices.
You're referring to PCIe 3.0 x16 cards, which typically support four drives using sixteen lanes. It's unclear if PCIe 4.0 versions are available yet, but it should become more common soon. I noticed some TRX40 boards already include PCIe 4.0 expansion options.