Ruby 4.0 introduces several key updates, including a new JIT compiler called ZJIT, improvements to Ractors, and an experimental feature for isolated namespaces called Ruby::Box. The release aims to enhance performance and usability without major breaking changes.

Ruby's JIT compiler, specifically ZJIT, enhances performance by compiling frequently used methods into native code while retaining their bytecode for safety and de-optimization. The article explains the mechanics of how Ruby executes JIT-compiled code, the criteria for compilation, and the reasons for falling back to the interpreter when assumptions are violated. Additionally, it addresses common questions regarding JIT functionality and performance implications.