Looking at asus's 4x nvme bifurcation card you cannot use an igpu enabled processor and maintain 4x4 with any CPU. I don't want to fill a slot with a dedicated GPU.
For a NAS I don't need 4 in a pool, just 2 per slot works fine with HDD backups.
I believe this has essentially been solved by the AM5 platform. The Ryzen 7000 IO die has a x16 slot which can usually be configured as 4x4, and has a tiny igpu which should be plenty for occasional server use.
The budget Ryzen 8000 CPUs are indeed quite limited, which is a shame if you wanted to go for an 8000G APU to use its powerful iGPU for transcoding.
For AM4: The ASUS card (at least the one I'm looking at) is PCIe4 4x4, while APUs only expose PCIe3. Should still work but over PCIe3 with X570. I've been running PCIe3 4x4 cards from other makers on that chipset and did not notice any issues.
Follow-up: Just learned that so far AM5 is actually a regression compared to AM4[0]. The best they can give is exactly 16 usable lanes (4 of total 20 reserved for chipset). Zen4c models only give you 10. Between this and Pluton I'm quickly losing interest in AM5...
[0]: OK, total bandwidth is still obviously up because we have gained a PCIe generation but still I'd have expected available lane-count to not drop.
AM5 is not a regression from AM4; it adds four more PCIe lanes. The laptop chips repackaged for the desktop socket don't have as many lanes, because laptop chips don't need that much IO.
For a NAS I don't need 4 in a pool, just 2 per slot works fine with HDD backups.