4.0.0 Fixes an issue with the WP Cron fallback where the $args passed to wp_async_task_add() were not passed back to the callback function as an array, and were instead passed as individual args to the callback function.

This brings gearman, rabbitmq, and wp cron clients all inline with each other, but if you have worked around the bug with the wp cron implementation in your code, this is likely a breaking change, and any workarounds will need to be removed.