This is a common error for the 4000 printer model. If a maintenance kit or fuserwas installed right before getting the error then pull the fuser out and check the sensor on the left side of the fuser.
It's possible that the sensor wasn't fully seated or got jostled during shipping so it sits at an angle and the sensor
flag will periodically catch on it.
Often, though, it's a paper jam in the front of the machine. Usually a piece of paper gets stuck in the front and when someone
tries to get it out some of the paper gets left in the machine, unreachable. If the paper is not stuck on the sensor flag,
it might start up and appear to be running OK until you print and the paper gets jammed in the front of the machine.
Sometimes, pushing through a more durable material like a file folder cut in half, will push the rest of the paper out.
To do that, remove the cassette tray and toner. Then take the file folder and slowly push it up from the bottom of the
machine to the top. Do not attempt to do this from the top to bottom or you could break the feed sensor flag.
Another option is to take the
MP pick up assembly
off and clean out the paper path to make sure all the debris is cleared out. Check for nicks on the plastic in the
feed path which can be filed down, if needed, so the problem does not recur.