I have just bought a used HP LaserJet 4L printer. It starts printing, but the paper sheet jams when it tries to turn at the back of the print, it cannot come out in the upper part of the printer. The jammed paper is printed well. What should I do? I have no users manual.

Empty cartridge maybe?
Otherwise it's simple: selt-test page(by printer button) printed ok - it's outer problem. Self-page is blank - it's inner problem. And so on... use Golden Section method

Remove the tonner and reinstall it, check the papers parth with a torch as the tiny bit of paper will cause a jam. Check for obstructions.

damnor01

Common issue with these older 4L printers. The problem is a small plastic gear on the main drive motor has cracked. This is what causes the noise and since the machine is not "running" there is no paper feed and thus the paper jam light. Basically there is no drive to any of the printers components. May be hard to find a replacement motor these days but that is what you would need to fix the printer.
wheatlandsvcs

You need the pick up rollers replaced. Call your service centre to replace pick rollers as these are feeding the pages crocked

When you removed the paper jam, was the paper torn? If so, did you ensure that you found all the pieces. (Put the piece of paper back together like a puzzle to verify). There could be a piece of paper stuck in the printer that is causing the paper sensor to fail.
The other possibility is that when you removed the paper jam, something became dislodged in the printer. Forcefully removing paper from the printer can cause this.

You need to replace the pick up roller. I did for my printer and it runs like new

You have a broken gear main motor. I had the same problem, bought $5 part and after the replacement of the gear,it runs like new

You will need to use the manual feed and insert the cardstock one page at a time. First look at the rear of the printer for a purple dial. Roll the dial downward. This will allow the printer to exit straight out the back of the machine. Allow enough room at the rear of the printer so the cardstock doesn't hit anything on the way out. Next look at the front of the machine at the **** just above the paper tray. You'll see an envelope icon. Align the cardstock when manual feeding one sheet at a time to the left side of the **** next to the envelope icon. When you hit "file, print" select MANUAL FEED in the print window.

The 4L will only do cardstock by one at a time in the way noted above as the feed system isn't strong enough to make the sharp bend coming out of the tray or in the back to bend it around and into the top tray.

Try lowering the toner density using the printing software or the printer menu.

Try and push the blue button after clearing the jam.