Roland discontinued the entire Edirol VST line years ago, including Super Quartet. The software was never open-sourced, and Roland does not offer it for sale or free download on any official website.
If you are looking for similar high-quality sounds legally, consider these modern options: Edirol Plugin Collection | Musical Artifacts edirol super quartet vst free download link
The Edirol Super Quartet is a nostalgic piece of early 2000s home studio history, but today you can get better sounds for free legally. If you absolutely need its specific character, look for a used license or consider upgrading to Roland’s current Zenology product. Roland discontinued the entire Edirol VST line years