I don't see version 400 on Esri's download page. Is that the version you
found? Here are a few links - most of the info is old, but it's the best
I've found:
https://resources.arcgis.com/en/help/arcgis-for-autocad/
http://giscadblog.blogspot.com/
https://www.youtube.com/channel/UCI0q3SW-F4ZZQmo9rYABgKw