A man was removed to Waterford Regional Hospital yesterday after he was stabbed in the city. He is reported to be in critical but stable condition.
It is believed that he was living in a hostel for homeless men in Lady Lane.
No motive has been established for the attack which occurred at 12.40pm on Olaf Street in the city centre.
The man was described by gardaí as being in his early 50s and Irish, although not from Waterford.
It is understood that he received a slash-type injury to the throat. It is not yet known what weapon was used in the attack and none was found at the scene.
It is understood that three men and a woman were arrested and detained at Waterford Garda station under Section 4 of the Criminal Justice Act 1984. One of them is believed to be a juvenile.
A technical examination was continuing last night. Supt John Sheehan of Waterford Garda station said that gardaí were carrying out searches at various buildings in the city.
Although a number of eyewitness statements were taken, Supt Sheehan is asking anyone who was in the vicinity of the attack to contact Waterford Garda station at 051 305300.