Passage time from four to two blocks in the voter model

We consider a voter model in which there are two candidates and initially, in the population Z, four connected blocks of same opinions. We assume that a citizen changes his mind at a rate proportional to the number of its neighbors that disagree with him, and we study the passage from four to two connected blocks of same opinions. More precisely we make explicit the generating function of the probabilities to go from four to two blocks in time k and we find the asymptotic of these probabilities when k goes to infinity.