Plasma 5.4.3 can be installed from the Kubuntu PPA. Users of open SUSE Tumbleweed get the latest KDE software shortly after its release. open SUSE Leap has a more conservative release cycle by default.
For details how to get new KDE software early, see this page.
Users can also get newest pre-configured KDE releases from the AOSC OS release directory.
Plasma packages are available in the [extra] repository.
Regularly updated ISO images are available on the Download Page KDE neon has the latest builds of Plasma built continuously from releases, Git stable branches or Git unstable branches. The daily builds, (to become 17.04, when released in April 2017) will have Plasma 5.8.4 soon.