1
2
3
4
5
6
7
8
9
10

chars = ['', *'a'..'z', *'A'..'Z', *'0'..'9']
chars.each |c1|
  chars.each |c2|
    chars.each |c3|
      chars.each |c4|
        puts c1+c2+c3+c4
      end
    end
  end
end