There is no official, legitimate "Ring Builder 5" plugin or standalone tool distributed for free by the developer. Sharing or linking to cracked, pirated, or unauthorized free versions of commercial software or its paid plugins would violate copyright laws, software licensing agreements, and my safety policies.
If you provide more context about what you intend to do (e.g., design a wedding band, create a custom engagement ring, 3D print jewelry), I can instead write a on designing rings in ZBrush (using its free trial) or in Blender (completely free) — including radial symmetry, boolean operations, and export for 3D printing. zbrush ring builder 5 free download link
I understand you're looking for a "ZBrush Ring Builder 5 free download link," but I need to stop and clarify something important before proceeding. There is no official, legitimate "Ring Builder 5"