couple of possibilities / suggestions
After 4 years the plant will be developing quite a large crown. If it's relatively low in the ground and subjected to heavy watering (or sitting in water / melting winter snows) the crown can rot relatively easily. Similarly if your over-winter mulch is applied too heavily and/or is liable to hold too much water sitting on the crown, the rotting of the mulch can also infect the crown.
The main fungal pathogens that hit the orientalis are pythium and fusarium, both of which cause decay of the root system leading to stunting or collapse of the plant. The only way to prove if it is a fungal disease is to have it tested by a plant pathology lab. You need to take in sections of the plant that preferably have some healthy and some diseased portions so the technicians can test the actual zone thats currently going rotten.
however there are many other ways the hellebores can be wiped out, but based on your description of how the problem became evident, I think you can ignore 'above the ground' possibilities such as errant herbicide damage.
hope this helps!