No, most brittlestars do not reproduce in this fashion. Most brittlestars that do reproduce by splitting have 6 arms. It is possible that it might survive it, but it is also possible it won't. That is some extreme trauma. Perhaps ate something too large. But I won't rule out acclimation shock. How long did you acclimate it? What is your specific gravity, ph and alkalinity? What are the other inhabitants? It still appears to be behaving relatively "normally" all considering, so it is not a loss yet.