Thanks for your sugggestions John. That is the site where I got the driver after talking with Applecare, but resetting the printing system with the option - key, along with some settings experimentation, seems to have done the trick! Prior to posting here I spoke at length with 2 different people at Applecare as well as HP, but they couldn't resolve it.
If anyone else has this great old workhouse of a printer (1999, no USB but has Ethernet) here's what worked for me:
Download the latest HP driver from the apple website John mentions. Reset the printing system with the option - key. The LaserJet 4050 doesn't appear in the Nearby Printers list. However I added it through its IP address (you can get this by printing out a Configuration Page from the printer's internal menu). Go to Preferences, select Print/Scan, and click on the IPP pane. Enter the printer's IP, set Protocol to HP Jetdirect-Socket and Use to HP LaserJet 4050 Series. The printer will set up. however in the Print Window dropdown (below Copies & Pages) "Unsupported PDE" appears and I get a low toner message (not true).
Thanks for the summary description Campy.
Upgrading from Snow Leopard or anything earlier sometimes results in previously configured printers not working. If they work, great, if not, you are better off wiping the slate clean by resetting the printing system.
Of all the printers that worked in previous iterations of Mac OS, there are very few that can no longer function with Mountain Lion. Every one I bought since 1985 still works.
That's encouraging! Just one more detail - I downloaded an older driver from the HP website which may have given me the functionality to use the printer's features (trays, duplexer unit)? It was for OS 10.2-10.5 From 10.6 forwad HP doesn't offer any driver downloads for this printer. Also I selected my printer's optional features in System Print & Scan interface. BTW, the best suggestion the Applecare folks made was to post in the Support Communities!
The duplexer function may work. You have to look in a Print dialog for the settings since everything has been rearranged.
First select Print from the usual File menu. If a Show Details button appears, click it. Then you will see a dropdown menu selection beneath the page orientation icons. Examining its options should reveal a selection for printer settings that will lead to paper handling, if the printer is so equipped.
The drivers incorporating these functions still originate from HP. Apple merely distributes the drivers now. If the duplexer option does not appear, only HP will be able to fix it in a future driver update.
... the best suggestion the Applecare folks made was to post in the Support Communities!
Good to know they're learning!