Using a PCIe 4.0 M.2 drive in a PCIe 3.0 motherboard?

[DataNalle]

My motherboard only supports PCIe 3.0. Because PCIe 4.0 is backwards compatible with PCIe 3.0 (at least this particular drive I'm looking at) my plan is to buy a PCIe 4.0 M.2 drive for future proofing as an incremental upgrade. When I eventually upgrade my motherboard (and other components) a couple of years down the line, I already have an M.2 drive that supports PCIe 4.0 speeds. I have an Asus Z170-A that supports NVMe M.2 PCIe 3.0 x4 Socket 3, with M Key, type 2280 (up to 32Gbit/s). Here are some other relevant components: https://pcpartpicker.com/list/hjm7Cz

 

For example a Samsung 1TB 980 PRO (MZ-V8P1T0BW) M.2 2280, PCIe Gen 4.0 x4, NVMe 1.3c looks nice.

 

The motherboard manual says "The M.2 socket shares SATA ports with SATA Express. Only one SATA device could be activated." which presumably means that I need to move my current SSD away from the SATA Express port into a regular SATA port to give all of the bandwidth to the new M.2 drive (which I of course plan on using as my OS drive)?

 

Additionally:

• What are some potential pitfalls of using a gen 4 M.2 on a gen 3 motherboard?
• Can I still expect regular gen 3 speeds in the 3500/3300 MB/s range with a gen 4 drive?
• I'm not entirely sure how this PCIe lane splitting thing works but that is something to keep in mind? I don't have anything else in my PCIe slots apart from the GPU so this should not be an issue?