HP LaserJet 4 printing vertical lines all down and across page

Sadly, the following HP support page offers about 8 dirrerent possible solutions, some of which you may have tried. You likely will have to try the others to see if they eliminate the problem.

Myself, I would download the HP Print and Scan Doctor just to see if it worked any magic or offered advice. Alternatively, Here is the support page related to printing issues with this printer: https://support.hp.com/us-en/document/c00599279

https://h30434.www3.hp.com/t5/LaserJet-Printing/A-Vertical-Strip-and-repeated-signs-on-Print-Outs/td-p/5668042

https://www.tonergiant.co.uk/blog/2017/05/black-lines-down-the-page-4-causes-and-how-to-fix/

Advisable for users & tech guys: Lift document feeder unit clean "scan head" glass with glass cleaner, check..

How old is the HP laser jet printer?

It is usually caused by the drum. The lines are vertical? perhaps the drum was scratched and needs to be replaced.

If you replaced the toner before the lines appears, the printer needs to be cleaned.

yes you have to catch printer making the line and then as a page is being printed you need to open door to stop paper after green cylinder(drum) as it is entering fuser, then you need to remove paper from printer without pulling through the fuser, if you see the line then rub your finger across line up and down page, if it smears at all then its the cartridge if not then the fuser

Does this happen both when copying and printing?
For printing solution, try another toner cartridge.
If lines continue to appear when copying (primarily through the automatic feeder) then lift the auto feeder, and clean the glass -mainly the narrow piece on the left side.

please rate solution :)

I think you need a new cartridge.

My first thought is probably what you've already considered as well...the LCD display is old and the functional elements are wearing out. My second thought was that it would probably be cheaper to replace than to repair. Nevertheless, I can try to address your question with a couple of ideas.

There are actually quite a number of "HP Laserjet 4" printers including the Laserjet 4, 4m, 4plus, 4m plus, and so on. Here's the list:

Product Search Results

Here's the webpage for the actual Laserjet 4:

Laserjet 4 Page

You should find a link to the manual for this product on that page. You can also ask a question directly on the HP support forums:

HP Laserjet Support Forums

Those options should give you your best bet on replacing the LCD if the option is reasonable or possible. Good luck.

Change toner cartridges again