Ruby 4.0.0 introduces significant features like Ruby Box and ZJIT. Ruby Box is an experimental feature that allows for isolated environments in Ruby applications. By setting the environment variable RUBY_BOX=1, developers can separate definitions, which is particularly useful for running tests with monkey patches or deploying web applications in parallel. This feature aims to handle dependencies and modifications without affecting other parts of the application. ZJIT is a new just-in-time compiler designed to enhance performance over the previous YJIT. Users need Rust 1.85.0 or later to build Ruby with ZJIT support. While ZJIT improves speed compared to the interpreter, it's still not as fast as YJIT, making it more suitable for experimentation rather than production use at this stage. Ractor, Ruby's parallel execution feature, has been improved with the introduction of Ractor::Port, which streamlines message passing between Ractors. This update reduces contention on global locks, enhancing overall parallelism. The release also includes various language changes, such as adjustments to nil behavior and updated logical operators for better readability. Core classes like Array and Binding received updates to enhance functionality, including new methods that improve efficiency and usability.