The command I needed was:
wmic memorychip get speed
This showed the speed of the internal 4GB RAM to be 2400.
I installed this:
Crucial CT4G4SFS824A 4 GB (DDR4, 2400 MT/s, PC4-19200, SRx8, SODIMM, 260-Pin) Memory,
and this has brought the system up to 8GB and everything seems stable.
I do not have the expertise to declare this is all working optimally but this is as close to a resolution I'm going to get and I hope anyone who has the same problem finds this useful.
I'll add Acer support are absolutely appalling and refused to give me any information on the RAM, other than incorrectly stating it was DDR3.
My follow-up complaint was just ignored.