Entirely my opinion but I suspect a knife similar to this being used..
1. The coroner said it was hard to refer to the wounds as puncture wounds because of how large the knife was
2. It was indicated the killer would be proud of the knife and may have paid money for it
3. Stabbing 4 people multiple times without your hand slipping down on the blade with the volume of blood, whether or not you have gloves on would be nearly impossible - In my opinion, the knife had to have a mechanism that kept it in place, e.g. a handle like most martial arts knives will have.
4. If the killer was skilled with the knife, a martial arts type knife might indicate they had experience in combat sports, thus they were able to kill all 4.