I have gotten the Legion Kill message many times and I know I have never killed 4 at once. Congrats, thats impressive. I'm not sure what you need to do for a Legion kill either.
I have also gotten the "Statue" feat except mine was during a Capture the Flag match on the Walls of Troy map. You get it by not moving for 5 minutes. I had the other team's flag and they had ours. I was at the ballista near our base and never saw another player from my team or the other team before I got the Statue notification. I forget why I ended up moving, it was either because someone from the other team came by or the other flag carrier died and I was able to return their flag to our base.
I've seen Legion Kill a few times and even asked Hawk of Rawk about it a couple days ago when we landed in the same match. I don't think it has anything to do with a multi-kill, because I got one last night after killing a loner on top of Daedalus Labyrinth.