Are SSD drives with PCIe 3.0 or 4.0 compatible with each other?

Cross compatibility

PCI Express versions are fundamentally backward compatible. PCI Express 3.0 is also referred to as "Gen3", 4.0 as "Gen4", and so on. A Gen4 or Gen5 SSD can be used in a Gen3 slot, and vice versa.

Performance comparison

When using a Gen4 SSD in a Gen3 slot, the maximum read/write speed of the advertised Gen4 SSD will adjust to the limitations of the Gen3 slot. Test runs with a 2 TB FireCuda 520 (a Gen4 SSD) in an older laptop with a Gen3 slot yielded the following results:

CrystalDiskMark 6.0.0Read (MB/s)Write (MB/s)
Seq Q32T13127.52981.3
4KiB Q8T81661.31758.3
4KIB Q32T1397.8324.8
4KiB Q1T143.79114.7

These values are at the upper limit of what is possible with a Gen3 slot. Similarly, if you install a Gen5 SSD in a Gen4 slot, the Gen4 slot will be fully utilized, even though the Gen5 SSD will not reach its full sequential read/write potential.

There is no downside to running a faster SSD in an older slot—especially since you may eventually transfer the SSD to a newer laptop in the future. Apart from the bandwidth difference, there are no disadvantages to using an SSD that does not match the PCIe generation of the slot.