PHP 8.4 brings a number of new features such as property hooks, lazy objects, asymmetric visibility, an object API for BCMath, new array functions, and a variety of other new functions and language features.
The PHP 8.4 release also brings a new JIT implementation based on IR Framework. There are also a variety of other performance improvements to find with PHP 8.4.
PHP 8.4 downloads for those building from source can be found at PHP.net . There is also the 8.4 release page that goes into more details on all of the great changes to find with the PHP 8.4 update.