You wouldn't want x to be a (numeric) column vector, because find() may not return a scalar. A numeric vector x can only put scalars into each x(i). However, a cell array is a possibility:
N=numel(b);
x=cell(N,1);
for i = 1:N %Edited typo
x{i} = find( a(:,4) == b(i) );
end
