It's actually C4123A.
Turn off the printer, remove the duplexer slot cover from the back
of the printer, slide the duplexer into place, turn the printer
back on. Configure your drivers to realize the printer now has a
duplexer.
Yeah, I saw that as the instructions for installing a different unit
(the q2439a) into a 4200/4300. But the 4050 doesn't seem to have a
duplexer slot cover...
If you plug in the corrected part number on HP's Support And
Drivers web page, it'll take you to a page where you can eventually
download a 4.7M PDF manual that tells you how to do the steps above
in sixty-eight different languages, and with pictures.
Unfortunately, that doesn't seem to be the case. Typing c4123a into
that page, and selecting "setup, install, and configure" yields four
options:
» HP LaserJet 5si, 8000, and 8100 Printers - Setting Up and
Installing the Optional Duplexer 7 May 2004
» HP LaserJet 4200, 4300 Series Printers - Installing the Automatic
Two-Sided (Duplexer) Printing Accessory 13 Apr 2004
» HP Color LaserJet 9500 Series Printer - Installing the Duplexer
9 Sep 2003
» HP Color LaserJet 9500 Series Printer - Installing the Duplexer
Needless to say, none of these are the c4123a.
The "manuals" link turns out to link to the q2439, which is a
different product.