Red Hat is planning to deprecate Plasma 4 on RHEL by 2024.
Plasma 4 was published in its LTS version for the last time in August 2015. The year before, Plasma 5 had been released, built using Qt 5.
Since then, a lot have happened.
Jan 2016, KDE announces KDE Neon
Feb 2016, openSUSE announces Argon and Krypton
Aug 2016, Canonical becomes a KDE patron
Jan 2017, KDE Slimbook is available
Jul 2018, Debian joins KDE's Advisory Board
Aug 2018, KDE Plasma runs on ARM Pinebook