Add buster-backports repo and install Roundcube 1.4 on Debian 10

Add buster-backports repo: echo "deb http://deb.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/buster-backports.list

Install roundcube: apt-get install roundcube/buster-backports roundcube-core/buster-backports roundcube-mysql/buster-backports roundcube-plugins/buster-backports

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information