New features:

• Enhanced performance - Executes queries 50% to 100% faster; builds schemas 20x faster
• Reduced memory requirements - as much as 75% less memory allocated per query executed
• Optional query caching service to cache document parsing and validation
• Custom deserializers can be written for GraphQL input objects
• Enhanced custom scalar support, including the ability to replace a built-in scalar
• Stricter type checking on scalars, both when read as variables or when being serialized
• Many bugs and feature requests related to scalars and variable parsing were resolved
• Applied directives
• Dependency injection extensions for the MS DI provider
• Extensive reorganization and clean-up of project internals, while retaining ability to customize behavior as desired
• Better support for validation rules that depend on directives or variable values
• Better documentation, including most of the public members now having xml comments
• Maintains target of .NET Standard 2.0; tested against .NET Framework 4.8, .NET Core 2.1, 3.1, and 5.0

See migration guide here:

• https://graphql-dotnet.github.io/docs/migrations/migration4

See main documentation here:

• https://graphql-dotnet.github.io/

See list of resolved issues and merged PRs for 4.0:

• https://github.com/graphql-dotnet/graphql-dotnet/issues?q=is%3Aclosed+milestone%3A4.0
• https://github.com/graphql-dotnet/graphql-dotnet/pulls?q=is%3Apr+is%3Aclosed+milestone%3A4.0