random_string.rb

class Array
  def rand
    self[Kernel.rand(length)]
  end
end

characters = %w(a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9)
Array.new(4).inject("") {|string, ary| string << characters.rand }