crackbabydumpster
Your cpu and mobo only support pcie 3.0 so you're not going to be getting pcie 4.0 speeds, at best the theoretical maximum for 4 3.0 lanes is below 4 GB/s. As for why it's so considerably lower than that, who knows, could very well be a crap ssd, could be set to 2 lanes, could be some other configuration issue or bottleneck. I would verify both M.2 slots are set to x4 in your BIOS setup if the option exists.