Precision tuning in the Dashboard — you can now set an engine's precision level using a slider in the Dashboard, with a description of how each level matches your content. See Precision.
Query-time precision — you can now override an engine's precision on an individual search request by passing a precision value, without changing the engine's global setting. See Precision for details.
User roles and permissions — Dashboard access is now controlled by user roles, with a read-only Viewer role alongside the full-access Manager role. See User roles and permissions for what each role can do.
Fixed an issue where re-saving an engine's schema (which happens on every Silverstripe build) could remove the semantic mapping and stop embeddings being generated for semantic search.
Silverstripe Search SDK 4.0.0 — the SDK now pulls in the latest version of each search module. The Silverstripe Search Administration module (silverstripeltd/silverstripe-bifrost-admin) is no longer bundled with the SDK. If you use the in-CMS administration module, install it separately with composer require silverstripeltd/silverstripe-bifrost-admin. This change is the reason the SDK moves to a new major version.