SecureRandom.hex(4)
=> "95bf7267"

SecureRandom.hex(4)
=> "95237267"

SecureRandom.random_number(a_big_number)

rand(9999)

def rand_by_length(length)
  rand((9.to_s * length).to_i).to_s.center(length, rand(9).to_s).to_i
end