From publisher blurb:
One of the drop in resolution sub-systems for StarCluster 3, In Harm's Way: StarCluster, and Commonwealth Space.
The Star20 Sub‐System uses 4d6 dice to play. All rolls are made with 4d6‐4, and the Sub‐system uses the Star20 Character Sheet along with the Character Generation Worksheet.
User note: freely available on the publisher website as a distinct file.