It’s marketing speak. 3D flash (stacked chips) with the control circuits stacked underneath instead of to the side. So it’s one louder.

https://www.tomshardware.com/news/sk_hynix-debuts-4d_nand,37...

Nothing - just marketing. It's their 2nd gen Periphery Under Cell (PUC) device.

Bigger number = more betterer

It'd have to be something involving time... maybe have each cell be a delay line? Or a resonant cavity, and store multiple bits at different modes? Not sure how either could be made small enough to be worth it though...