Perfect, I can answer this.
I literally just bought the Slash 4x4 Ultimate last week. I bought this battery. It fits perfect, and I mean, just a hair bigger and it wouldn't fit. The power is great, reliable, and barely gets warm at all. From the lipo brands available on amazon I trust HRB the most because I used to run their huge 6s lipos in my EDF jets without a problem (and those would suck power at a constant 200a or something crazy like that). Other lipos I really like, Glacier ICE from BuddyRC, Turnigy Graphene from Hobbyking, Admiral from MotionRC, GensAce, and Tattu.