I know 3.9.4b is newer than 3.9.2. My point is that, unlike 3.9.4b, in the case of 3.9.2 there is an official Portage ebuild. Thus, in the case of 3.9.2, the patching and compiling, installation of dependencies etc. is all done automatically and precisely by a proper script ('ebuild') created specifically for use by the Gentoo package manager (Portage). It is therefore more likely to install the application (including all dependencies) correctly on your PC than a manual procedure from a third-party (i.e. non-Gentoo) Web site.
If you sync with the latest version of the Portage ebuild repository (that's what the emerge --sync command does) and then take a look at the hplip-3.9.2 ebuild using a text editor, you'll see that the ebuild script does a heck of lot of things to install the hplip package. The ebuild file on your PC is /usr/portage/net-print/hplip/hplip-3.9.2.ebuild, so open it with your favourite text editor and you'll see what I mean. An ebuild created by a Gentoo developer for the Gentoo package management system is worth trying, even if the version of hplip it installs is a little bit behind the latest version of hplip.
Over and above the aforementioned reason, the latest version of a package is not necessarily guaranteed to work. I find this happens quite often with Linux, and I sometimes have to try, or revert to, an earlier version. If you look at some other threads in the SL forums you'll find examples of this. For example recently I had to revert to an earlier version of udev to get Bluetooth working again on my laptop, I had to revert to earlier versions of KNetworkManager and NetworkManager to get networking to work again, and so on.
I'm not saying that installing hplip-3.9.2 using the official package manager rather than installing a later version manually outside the package manager is guaranteed to solve your problem, but it's definitely one avenue that any Linux user should explore. In general, it is only recommended to install software manually without using a Linux distro's package manager as a last resort. Package managers were created for a reason.