80% loss of smell after open rhinoplasty 4 years ago, what could have gone wrong during surgery?

Loss of smell after rhinoplasty may be due to obstruction in the nasal cavities or a fracture of the cribriform plate through which olfatory nerves run.  If your breathing is not obstructed, I would consider the latter as a cause.  Unfortunately there is not much to be done for this.  The fracture may have occurred during the septoplasty portion where the septal cartilage is harvested.  The ethomoid and vomerine portion of the septum is fractured in a controlled fashion but the fracture line may run up to the cribriform plate.  Did you notice any clear drainage from your nose after the surgery?  

After rhinoplasty it is possible to lose the sense of smell although it is uncommon.  Sometimes just the nature of having a general anesthesia can create this effect although that is even less common.  The most likely explanation is both agressive external bony maneuvers in conjunction with aggressive septoplasty in the bony vomerian component of the septum which can damage the cribriform plate and thus the olfactory nerves.  Regardless of cause, at 4 years from surgery there is no effective way to improve the problem.  It is now likely a permanent issue.  The positive is that you still have some sense of smell which is fortunate. 

Dear Annon3705,It is not uncommon to lose sense of smell immediately after septorhinoplasty due to loss of air flow into your nose. However, I would expect this to resolve gradually over time. I would suggest that you return to your surgeon and discuss your concerns. I suspect the decrease in air flow into your nose may be contributing to the problem and not anything more serious.

As you mentioned, you had your surgery four years ago, and while rhinoplasty can affect olfactory function, this change is often temporary, with many patients experiencing a return of their sense of smell over time. If you have undergone a CT scan or MRI that shows normal results, that is reassuring.
It is also important to consider whether your surgery took place during the COVID-19 pandemic. If so, there is a possibility that you may have contracted COVID-19 shortly after your surgery. Anosmia, or loss of smell, has been reported in approximately 50% of patients with COVID-19, with around 10% experiencing permanent anosmia.