Does a PCI 4.0 card work with a PCI 3.0 slot?
If I upgraded the motherboard, would I have to get a new copy of win 10?
If I upgraded the motherboard, would I have to get a new copy of win 10?
Yes.
If you tie your Windows 10 license to a Microsoft account you 'should' be able to use it with your new motherboard.
Current GPUs don't even use all the available PCIe 3.0 bandwidth.
NVMe SSDs can benefit from PCIe 4.0 but it doesn't really make a difference in normal, everyday computing, only synthetic benchmarks.