"Ghost" Printing

Sounds like a fuser problem to me. Can you measure exactly how far apart the ghosts are. This will help identify the roller that is the problem.

Here is a link with defect spacing for a 4050.

http://www.printertechs.com/tech/repdef/ruler4000.php

Most common causes :
Dirty fuser hot roller
Wrong toner or cheap copy toner.

Definitely a fuser - We have a large amount of 4050N's in the company, and I've had to deal with similar issues plenty of times. The fuser unit in the HP 4050 is very easy to replace (2 screws in the back of the printer and the unit slides out). Always remember: BUY ORIGINAL PARTS!!

Alright, I measured the distance between the original printing and the ghost... it's 3 3/4 inches. That's not listed on that ruler joed linked to...
I'll try replacing the fuser, tho. how much do those things cost, anyways?

A 4050 fuser should cost around 60-80 GBP, not including installation (which, again, is very simple).

Correction - could even be as much as 100 GBP. If you don't mind a little extra expense, go for a complete Maintenance Pack which includes a complete set of rollers.

If the Ghost is that far down, it could be the image drum is not cleaning off .  (the fuser roller is usually smaller)
To diagnose this, print something, and try and stop it half way through the printer (before it enters the fuser). If the image is repeated before fusing, it is the image drum.
Again, this fault can be caused by incorrect or third party toners.

what the heck is the fuser, and where is it located? I can't find anything in the HP manual...

The fuser is the "hot bit" where the the tomner is melted onto the paper.
It is usually the last part of the paper path before the paper tray.

And actually, your problem is more likely to be the image drum ( sometimes part of the toner unit, sometimes a seperate unit).

Are you using HP toner?

Before you attempt to replace or just mess with any internal part (excluding the toner unit), make sure you unplug the power lead. The current in some parts can be as high as 10-15 amps!!
If you look at the printer's rear, you'll see, two detachable plastic covers. Behind them you'll find the two screws that will enable you to remove the fuser unit.
The image drum, as wyliecoyoteuk said, is indeed part of the toner unit, so it's best to first try to use a different toner. If the problem persists, the fault is with the fuser.

Alright...I've been waiting for the printer's owner to go out and buy a new toner cartridge, that's why there's been that long pause. Anyways, he just bought a new toner cartridge (not HP...sigh...) and the problem persists. I haven't gotten ahold of one of the new printouts, but I think the ghosting distance is the same...3 3/4 inches.

Could it still be the image drum?

Also, I found the fuser, but I was unable to remove it. I got the screws loosened, but it didn't seem to want to come out. Is there some sort of trick, or do I just need to force it and hope for the best?

Here is the fuser removal instructions for a 4000/4050. You just need to pull it out.

http://www.printertechs.com/tech/mkinst/mk-4000-2fuser.php

Sounds like a fuser. But if you run a half-page test it will determine whether its the toner or fuser. Here's how you test: insert blank page into manual feed - print a page- wait until the page is halfway printed- open the cover- remove the page (be careful toner will smudge since the job is before the fusing process). If you notice the ghosting here- it was the toner.

Were you able to remove the fuser? If so most likely you're able to see the damaged roller.

if you look at the spacinbg list, 3 11/16 inches is drum unit, which is close enough to 3 3/4 inches as makes no difference.

I would suggest that the solution, as stated above, is that the fake supplies are the problem,  it seems that the questioner is unable to change that, so has abandoned the question.