That looks more like hardware fail, but could just be toner. The spots remind me more of hardware problems though. The 4100 is an older laserjet so I am sure it has gotten used a lot. The middle looks like it getting non fused toner on the paper, then it just smears as it goes through the rollers. For printers, if you don't want to pay for professional service, I start with the cheapest things.
- Re-install drivers on clients (Does it print like that from one machine or multiple?)
- Pull the toner and clean the printer up. (Be careful, they can get very dirty and you don't want to inhale the toner)
- Replace toner pack or try a different known working one(I recommend you do not use remanufactured ones)
- Beyond that, it could be the fuser module (toner not getting fused to paper and smearing) or need a drum kit. The printer test page should give a readout of the total pages printed and some of those hp's give percentages of drum and toner life. Those are not cheap parts (>$100) however and this is where you might want to consider a new printer.
In the end, that printer is an older model and a new printer purchase might be a better use of funds.
I kind of doubt it is your software if it is just one client. Bad print drivers or a dying printer is more likely the case. And it is only your software when this kind of printing is experienced?