I think you may be biting off more than you can chew here.
First, what is the purpose of this card? Additional memory...for what? It can be interfaced over PCIe, true, but you'll never see the type of latency performance you could get from real DDR4 attached directly to the CPU memory controller. And why does it need to be DDR4?
Perhaps latency is less of an issue, and you can tolerate increased latency in exchange for having 1TB of volatile, RAM storage. You would have to use a FPGA to act as a PCI Express device that is connected to all this RAM. You are straight into Virtex-7/UltraScale or Stratix 10 territory, the flagship FPGAs from Xilinx and Altera respectively, each of which costs thousands of dollars a piece. They're the only parts with enough high-performance I/O to support what you need.
If you can find one of these FPGAs that can support the full amount of memory you need to interface too, great! Otherwise, you'll likely need to use a PCIe packet switch from the likes of PLX or let you strap several FPGAs behind a single x16 PCIe 3.0 link to the host machine. On these FPGAs, you'd have to instantiate the memory controller IPs, and develop / write an interface to go from RAM to PCIe.
So, this device is technically possible (the best kind of possible), but would be insanely expensive, power hungry, honestly not that fast, and potentially have issues fitting onto a full-length PCI Express card. Remember that you'd need to design a power supply to feed tightly regulated sub-1.0V core voltage at 10s of amperes to each FPGA.
This has been done on a smaller scale (maybe a few GB of memory) attached to a FPGA via PCI Express for acceleration of cloud applications, but I don't think anyone has taken it to the large amount of memory you want.