What Version of hplip do you have installed?
According to this website:
http://hplipopensource.com/hplip-web...00_series.html
the printer is supported since HPLIP version 2.8.5.
If you use the package from provided in Slackware current ( Hplip 3.9.2 ) remember to install cupsddk-package as well because ppd-files aren't static anymore but will be build dynamically.
Using this package, cups gave me an error like: "foomatic-rip-hplip missing - no such file or directoy" which I could solve by downloading Slackwares source package, editing the slackbuilds script to enable everything with foomatic and rebuild it.
Now my OJ j4680 works like a charm.
Hope this helps ...