RabbitMQ 4.0.0 is a major release with new features and improvements. The Khepri schema data store has matured, AMQP 1.0 is now a core protocol, and the AMQP 1.0 implementation is more efficient. Quorum queues now support priorities, and classic queue mirroring has been removed in favor of quorum queues and streams. RabbitMQ 4.0.0 requires Erlang 26.2 and upgrades from 3.13.x, with mandatory feature flags. There are changes to default settings, such as the message size limit being reduced to 16 MiB. The release also includes bug fixes and enhancements for AMQP 1.0, quorum queues, and other plugins.
https://github.com/rabbitmq/rabbitmq-server/releases/tag/v4.0.0