TLDR :ST16000NM002G belongs to the X16 platform and ST16000NM004J belongs to the newere X18 platform with better transfer rates.
Seagate is discontinuing the 16TB drives on the X16 platform, which is understandable since they had 16TB on both the X16 and X18 platforms and that duplication had to go at some point. X18 and X16 are almost identical in terms of cache, MTBF and warranty, etc., but the transfer rates of X18 are slightly better. Since X18 is a newer platform, buyers should go for X18 if it is compatible with their device.
Here is a table showing Old and New part numbers
|Old Part Number (X16 Platform)||
New Part Number (X18 Platform)
|ST16000NM001G||ST16000NM000J||16TB Standard SATA||X18 has marginally better transfer rate|
|ST16000NM003G||ST16000NM001J||16TB SED SATA|
|ST16000NM002G||ST16000NM004J||16TB Standard SAS|
|ST16000NM004G||ST16000NM005J||16TB SED SAS|
|ST16000NM009G||ST16000NM007J||16TB SED-FIPS SAS|