NVMe PCIe x4 SSD on M.2 PCIe x2 slot?

Solution 1:

A single lane of PCIe 3.0 is capable of, in theory, 985MB/s. Two lanes would be twice that at 1970MB/s.

Overheads may eat into that slightly.

PCIe should also negotiate how many lanes are available, so if only two are available then both sides should fall down to using two lanes. It should work fine. An x4 SSD might be able to theoretically reach up to 3940MB/s, but in practice many of them are closer to 2-3GB/s and whether or not you can actually make use of that bandwidth is down to the program itself.

Okay it will be slower than being in an x4 slot, but probably not noticeably so. It will still be far faster than a SATA SSD by nearly 4 times, and an order of magnitude faster than an old HDD.