Why and how the Armenian Orthodox Church came to have four Catholicoi is beyond my precise knowledge, but I believe the answers to such questions are inextricably tied to the rather complex political history of the Armenian nation.
What I can say with absolute confidence though is that the Armenian Orthodox Church is ultimately one local Church. The fact there are four Catholicoi does not challenge the unity of the Armenian Church; I do not believe there is any tension or divisiveness amongst the various jurisdictions within the Armenian Church; they co-operate synergically and in unity. I believe the Catholicos of Echmiadzin, who is otherwise known as the Catholicos of Armenia and of All Armenians, is the dominant Patriarch with universal jurisdiction over the Armenian Orthodox Church.
Salpy is probably in a better position to clarify and elaborate upon the matters in question; she is also, ofcourse, free to correct me if i've spoken ignorantly.