simply put pci 4.0 double through pcle 3.0. the two standers are structurally very seemlier with the key different benign the higher transfer rate and some cases the martial uses on achieve suspenseful.
Yes it will be limited. PCIe 3.0 has 8 GT/s data transfer rate. PCIe 4.0 has 16 GT/s rate. While PCIe 4.0 is backwards-compatible with the 3.0, you will only be getting the 8 GT/s rate.
absolutely yes, you can! The design of the physical slot itself has not changed, and any Gen 3 device, such as a graphics card or sound card, will continue to work in a Gen 4 slot.