Based on the released metrics this is the only memory I'm aware...

Based on the released metrics this is the only memory I'm aware of that is / was even close to 4DS
https://www.eetimes.com/unity-rolls-storageclass-memory-technology/





They went into a development agreement wit Micron in 2011 - then Rambus moved on then in an acquisition in 2012
https://www.rambus.com/rambus-acquires-unity-semiconductor/

all these years of no metrics from 4DS now seems to relate to this entity being known about and Microns' interest in the process technology as i understand it - don't know what Rambus did with it but it's not in the market place - former head of Unity semiconductor Dave Eggleston may may have an idea

the only competitor I know about with the 'same' process technology and no longer on the radar