Hi, I’m next update can you also update roundcube to latest version 1.4 beta?
We try to track the latest stable releases of the products we use — so, not betas.
I understand. Can I modify installation to use beta instead before release?
You can modify anything on the system, it’s yours! 
BUT modifications are not officially supported and only few will support modded servers.
That being said, it’s just roundcube which can be configured to run on any server btw - you can run roundcube beta on an external server and have roundcube load data from MIAB via config.php