I've been searching the main KDE site and this forum in an effort to discover which version of the Plasma workspace I should be using for production use: 4 or 5?

Unfortunately there is nothing obvious. So perhaps I should just ask. What is the current recommended production version?

I suppose that may depend on what precisely you mean by for production use.
KDE 4.14 is certainly stable, but isn't updated much anymore (no new features, not sure about bug fixes or security updates - may depend on your distro?).
I've heard that KDE 5.6 and 5.7 are supposed to be quite stable and usable for most people.
Another factor which may play into the (perceived) stability of the desktop is the choice of (version of) distro. At this point you are probably only going to find KDE 4 available in LTS or older distros like Ubuntu 14.04, Linux Mint, PCLinuxOS, etc. while most newer versions of various distros will provide 5.x, including Neon (built on Ubuntu 16.04), Fedora, OpenSuse, Arch, etc.

My suggestion is to try them both and see for yourself.

airdrik wrote:I suppose that may depend on what precisely you mean by for production use.
KDE 4.14 is certainly stable, but isn't updated much anymore (no new features, not sure about bug fixes or security updates - may depend on your distro?).

Plasma Workspaces 4.11 (the last version of Plasma based on kdelibs4 - there is no Plasma 4.14) is out of support by kde.org. Distributions can provide their support if they ship it.

airdrik wrote:I've heard that KDE 5.6 and 5.7 are supposed to be quite stable and usable for most people.
Another factor which may play into the (perceived) stability of the desktop is the choice of (version of) distro. At this point you are probably only going to find KDE 4 available in LTS or older distros like Ubuntu 14.04, Linux Mint, PCLinuxOS, etc. while most newer versions of various distros will provide 5.x, including Neon (built on Ubuntu 16.04), Fedora, OpenSuse, Arch, etc.
My suggestion is to try them both and see for yourself.

As you mentioned, Plasma 5.x is available with the stable (and unstable) version of distributions released more or less after 5.2.