Are SSD drives with PCIe 3.0 or 4.0 compatible with each other? Cross compatibilityPCI Express versions have always been backward compatible by design. PCI Express 3.0 is also referred to as "Gen3u201d, 4.0 as "Gen4u201d. A Gen4 SSD can be used in a Gen3 slot and vice versa.Performance comparisonWhen using a Gen4 SSD in a Gen3 slot, the maximum read/write rate of the Gen4 SSD will drop to the conditions of the Gen3 slot. Test runs with a 2 TB FireCuda 520 (a Gen4 SSD) inside 2019's XMG FUSION 15 (laptop with Gen3 slot) resulted in the following, very good values: CrystalDiskMark 6.0.0 Read (MB/s) Write (MB/s) Seq Q32T1 3127.5 2981.3 4KiB Q8T8 1661.3 1758.3 4KIB Q32T1 397.8 324.8 4KiB Q1T1 43.79 114.7 Aside from the difference in bandwidth, there is no reason not to run a Gen4 SSD in a Gen3 slot - especially since you can move the SSD to a newer laptop at some point in the future to make full use of its bandwidth potential.