How many more peoples have to die to you take a action?
Again, why trusting someone and killing other people without proof of guilty?
I think i got your point. In your story it is verfied that 3 of 4 shoemakers are the murderers! Right? The problem is only to separate the innocent one.. Well in this case i would isolate(imprison) all 4 from the rest of the city until time comes up with a solution to find the innocent one.