Drum. To much toner is always the drum. The drum is electrically charged (positive I believe) and the laser prints a negative charge on the surface. The toner has the same charge as the unexposed drum and so only sticks where the laser exposed it. This is then transfered to the paper by pressure contact. All the fuser does is melt the toner into the paper. It doesn't add or remove toner. If the image is correct on the paper but smears or flakes off, then you have a fuser problem. The problem you describe can be caused by wear on the print drum, a light leak that is exposing the drum, or probably some other things I no longer remember.
Most printers these days include the print drum in the toner cartridge, but some older printers didn't. The drum was a seperate replacable item, mainly because it was really expensive. Don't remember how the 4L is laid out. I haven't worked on one since the mid 90s.
This isn't quite true. The toner is transfered to the page not by pressure but charged transfer roller which always lives directly opposite the drum in the paper path. The page travels between the transfer roller and the drum and the charge electrically 'pulls' the toner off the drum and onto the page.
Also, too much toner can be caused by many things besides a faulty drum, including a faulty fuser. Toner can be left on a faulty fuser from previous prints and then be transfered to the next print as the page passes through or even onto the same page causing ghosting of the image down the page. A faulty transfer roller or HV PSU can also cause excessive toner.
Finally, there are many, many modern printers that still use a separate drum and toner unit. Loads of Brother printers, Kyocera, samsung and Oki to name a few.
However in this case I'd fully expect it to be the drum/toner cart.
If we cannot be free, at least we can be cheap! FZ.