It depends on the PC. If it's a mini ITX board with an unused 16 lane PCIe slot, you can put an adapter in there with 4 NVMe drives. Make sure the motherboard supports PCIe bifurcation though.
Another option is an M.2 to SATA adapter. They will connect 4 to 6 drives to an M.2 slot. Finding a place to mount those drives could be tricky though.