Prints pages with dark background

SOURCE: grey background and dark streaks

Change out the toner first and then the drum. Also be sure there is not excess toner underneath where the toner is installed. If so clean that out with a dry cloth.

SOURCE: Dark Background on HP Laserjet 4100

Several Options:
- Increase the Toner Density Level to 5
- Use freshly opened media
- Use a new print cartridge, if near end of life
- Run a cleaning page to clean the fuser

SOURCE: Grey background over entire page.

If toner is getting onto the transfer belt, it can only get there by either being put there intentionally, or leaking.

Failure of Transfer Belt is normally degraded print quality - usually poor density, or a failure of the support behind the belt (usually puncturing the belt), and scoring a drum.

SOURCE: When printing 2 sided sheets our machine puts dark

When you are printing 2 sided sheets the reason you are getting streaks, is your first sided not dry yet. I know it is a laser printer, even laser you have waite a bit.

SOURCE: dark black streak down the middle of the page

sounds like your cartridge is leaking. I'd try replacing the cartridge