So, it's a bug?

I tried on slackbuilds.org but that is only showing me hplip-plugin-3.16.5 from 14.2.

It's only 21Megs, which is a relief. I think I was 2½ - 3 hours building qt4! I've a twin core i3 laptop.

I can do the configure, make, package & install it, but if you link me to the slackbuild you guys use, I'll edit that and use it. It will have to be tomorrow anyhow, because it's late here

I'm not sure if it's a bug, that's why I'm having you try and build this. If you build it with explicitly disabling qt4 support and the problem goes away, then we can suggest to Pat to change it on the official script. If it doesn't work and it is still referencing PyQt4, then we'd need to check if there's some other option that needs to be disabled or file a bug report upstream.

The code I provided will download everything needed to build the hplip package. It will include the source, SlackBuild, slack-desc, and several patches Pat uses. Once downloaded, cd into the directory, edit the SlackBuild in there, then simply run the SlackBuild. The hope is building this with explicitly disabling qt4 will ensure that no portions of the package rely on qt4 components.

We don't want to build qt4 or anything relying on it, as -current has moved beyond qt4 and we don't want anything in the OS relying on it.

OK, I built hplip with --disable-qt4 just after --enable-qt5 as you suggested. Hp-check still bales out in exactly the same spot.

I did a little digging. hp-check is a symlink to a script /usr/share/hplip/check.py, which is not compiled. In trying to satisfy the slackbuild, I made a tar.xz and had a look there. A grep for qt4 rolls off the page. In my (not hugely educated) opinion, it would take a significant patch to remove references to qt4. You could of course patch /usr/share/hplip/check.py but that might simply hide other problems. I don't know how a recursive sed command would work, simply swapping qt5 for qt4, but that sort of unprofessional thing usually ends in frustration. Mind you, it did compile with --disable-qt4, so the binaries might be ok.

I have Mint-19.3 (= Ubuntu-18.04 with go-faster stripes) and Debian Buster in the form of RazPi OS (32bit), and I installed the sane packages and dependencies on them both. As my Printer/Scanner is networked they can easily find it, but both threw the same error as Slackware. So I have my result- a speculative diagnosis confirmed. From the Slackware POV, you have yours - there's a lot of qt4 references in hplip, and hp-check is broken.

EDIT: I had a look in check.py, (my error shows on line 303) and it references some external lists, checks them one by one, and exits on this pyqt4. It will take someone who knows python (i.e. not me - I'm a hardware guy) to sort this.