function z = norm4(A,Nsamples)
% NORM4   z = norm4(A)
%   computes ||A||_4 by random vector generation
%   examples:
%     >> norm4(diag([-3 2.5 3.11 3.09]))  % should be exactly 3.11

if (nargin < 2), Nsamples = 100; end

[m,n] = size(A);
z = 0;
for j = 1:Nsamples
  x = randn(n,1);
  z = max(z, norm(A*x,4)/norm(x,4) );
end