What’s New in Magento 2.4?

Security enhancements : 

  •      Two-factor authentication (2FA) is now enabled by default for the Magento Admin.
  •      Template filter strict mode is now enabled by default.
  •      Data rendering for UI data providers is now disabled by default.
  •      New \Magento\Framework\Escaper class
  •      Support for new security.txt file

Platform upgrades

  • PHP 7.4 support introduced and PHP 7.1 and 7.2 deprecated
  • Support for PHPUnit 9.x and deprecation of PHPUnit 6.5
  • Elasticsearch 7.6.x support
  • MySQL 8.0 support
  • Removal of the MySQL catalog search engine
  • MariaDB 10.4 support
  • Migration of dependencies on Zend Framework to the Laminas project
  • Removal of the core integration of the Signifyd fraud protection code
  • core Braintree module has been removed from the code base

Infrastructure improvements

  • Removal of core integration of third-party payment methods
  • Support for partial-word search for Elasticsearch (new default search engine)
  • PayPal JavaScript SDK upgrade
  • Deprecation and removal of the Web Set Up Wizard
  • Composer update plugin
  • Seller-assisted shopping

Performance improvements

  • Improvements to customer data section invalidation logic
  • Multiple optimizations to Redis performance. 
  • Improved caching of results of SQL queries to inventory tables
  • Improvement of up to 25-30% to Quick Order add-to-cart performance.

Other 

  • Adobe Stock Integration v2.0
  • New Media Gallery
  • Inventory Management
  • GraphQL
  • PWA Studio

Other, for more information refer Magento official devDocs