The description is not very clear. You need to get 4 kills in a single normal attack. Not using any abilities. I did this with a 2 handed weapon. Not sure it is even possible with another type of weapon.
I waited until Level 22. This meant I was stronger, and I had a powerful weapon (WDPS 1023). I then loaded up the Hunt Wolves map at the boat and gathered up enemies. Each swing would kill everything it hit, so I just had to hope I got 4. It took a few tries, but I got it eventually. The bonus blood box that appears will state multikill if you go 4 kills.