In a presentation, SK Hynix has talked about its next-generation NAND chips. The new NAND will base based on 96 layers (tlc) flash, stacked on top of the circuits, SK Hynix calls that 4d-NAND. With 3D NAND, the supportive circuitry was placed next to the flash chips.
The big advantage here is that they can downsize the entire package, that results is cheaper to fab production and you can fit more of these chips into a PCB. So the idea here is large volume SSDs, at a lower cost. Arguably the 4D naming is a bit tacky. The new chips are equipped with 1Tb-dies for SSDs and 512Gb-dies for integrated storage. The company also mentioned working on 4d-NAND with QLC technology, which should arrive in the second half of 2019.