Index exceeds matrix dimensions.

for f=1:m
    for i=1:m
         for j=1:4 
             di=abs(X(f,j)-X(i,j));
             Z(i)=di;
         end
         [B,I]=sort(Z);
         for c=1:20
            if(X(f,5)==X(I(c),5)) 
         k=0
  end
  end
  end
  end

[KSSV]

With out the complete details of variables, it is tough to answer. I already commented in your previous question about the error.