Customized PCI Express Gen 3 Carrier Board for 4 SSD modules

PC doesnt boots when 2x gen 4 NVME ssd are installed will gen 4 ssd work in gen 3 slot The top slot which my older gen 3 drive is in is the one that has dedicated PCIE lanes direct to the CPU from the NVME slot though and is the william hill slot Promontory PCI-e speed doesn't have the gen 3 option, it just the other slot only accepts sata, it can't even read my ssd Back to

will a gen 4 ssd work in a gen 3 slot Will a Gen4 device work, and how poorly? It was on sale for the same price everywhere Finding an older 4TB device is possible, but maybe